Even in light of this very important nuance, Ehrman's argument runs into problems precisely on the issue of identifying anti-adoptionistic variants (whether in terms of intention or function). For example, he assumes that references to Joseph as Jesus' father could be read as implying that Jesus was not (yet) God's son, and so changes to these references represent "anti-adoptionistic corruptions." But this assumption runs into problems very early on. In his brief discussion of a Syriac translation of Matthew, Ehrman points to the variants at 1.16 and 25:
The fifth-century scribe of this manuscript was either thoughtless in the extreme or somewhat inclined to see Joseph as Jesus' actual father, for he concludes Matthew's genealogy of Jesus with the words "Jacob begot Joseph; Joseph, to whom was betrothed the virgin Mary, begot Jesus, who is called the Christ" (Matt 1:16). Similarly, the following pericope ends not with the statement that Joseph "had no relations with her [Mary] until she bore a son," but with the curious observation that Mary "bore to him [i.e., Joseph] a son" (1:25). (54)
To be sure, Ehrman does not read this as an anti-adoptionistic corruption; instead, the variants at 1.16, 25 "were produced from carelessness rather than intent" (54). He concludes his discussion of this particular scribe's carelessness: "Since there is almost no reason to construe any of the manuscript's variant readings as original in these cases, once can only conclude that the scribe was simply inattentive to the doctrinal ramifications of some of his changes" (55; my emphasis).
Notice that Ehrman assumes here that variants "have" doctrinal ramifications; the readings in question, by themselves and (almost) ontologically, support a certain theological position even though Ehrman doesn't attribute this position to this particular scribe (remember, the scribe was careless rather than theologically motivated). After all, this same scribe apparently chose not to "eliminate the word 'virgin' (παρθένος, v. 23) or to modify the clear statements that Joseph had no sexual relations with Mary" (55). Since the variant readings at Matt. 1.16, 25 "have" their particular theological significances, the scribe responsible for these readings was either (i) advocating an adoptionist christology, or (ii) unaware of his changes' adoptionistic import. This scribe "appears to have understood his own formulation—if he understood it at all—in the sense that Joseph became Jesus' father (through adoption?), although he was not his actual father" (102, n. 48; my emphasis).
But there's a third option for understanding this scribe's admittedly peculiar transcription. (iii) Even a statement as seemingly clear as "Joseph . . . begot Jesus" do not mean what we think they mean, i.e., that Jesus was conceived via a sexual act between Joseph and Mary. Don't misunderstand; a moment of carelessness and conformity to a pattern (x begot y) may very well explain this particular text. But note that, even with the strange reading, "Joseph . . . begot Jesus," 1.16 still breaks with the regular pattern "x begot y" to explain that Joseph was betrothed to Mary. And in the very context of claiming "Joseph . . . begot Jesus," the text affirms Mary's virginal state at conception/begetting! For this scribe at least (and presumably he was not completely idiosyncratic!), Joseph "begtting" [γεννάω] Jesus did not preclude either the virgin birth or, by implication, Jesus' status as God's s/Son.
At other places Ehrman mentions variants that I simply do not see as directly relevant for the adoptionist controversy. Notice, for example, the following:
The text of Luke 3:23 would presumably have caused orthodox scribes few problems, since it explicitly states that Joseph was not Jesus' real father, but was only "supposed" to have been. Nonetheless it is striking that in two of our Greek witnesses (W 579) the genealogy of Joseph that follows is deleted altogether. It is difficult to judge what may have led scribes, either those of our manuscripts or those of their exemplars, to omit some fifteen verses from their text; but perhaps they recognized the incongruity of tracing Joseph's ancestry back to Adam in a story about Jesus, when Joseph was in fact not Jesus' father (as the text of v. 23 itself indicates).
Here Ehrman reads the alteration of a useful text for anti-adoptionistic readings as (potentially) an anti-adoptionistic corruption. This strikes me as having his cake and eating it, too; if Luke 3.23 did not equivocate on Jesus' relation to Joseph [ὢν υἱός, ὡς νομίζετο, Ἰωσὴφ], and if these two Greek witnesses excised Joseph's genealogy, then this would clearly be an anti-adoptionistic corruption. The fact, then, that "some modern scholars have seen in the genealogy an implicit challenge to the notion that Jesus had no earthly father" (56–57) does not justify reading W and 579 as anti-adoptionist corruptions. So "some modern scholars" read the Lukan genealogy as potentially adoptionistic; did anyone int he ancient world ever read Luke 3 this way? For his argument to work, Ehrman needs to adduce evidence that specific variants did function anti-adoptionistically (e.g., see Luke 2.33), not merely that specific variants can so function.
Finally, this brings up a second methodological problem with Ehrman's argument. Ehrman focuses so clearly and so intensely on individual variants that he does not address whether a particular manuscript exhibits anti-adoptionistic tendencies.1 It's one thing to show that a given manuscript makes a change at Luke 2.33 and/or 2.48 (see pp. 55–56); it's another to show that that manuscript consistently clarifies ambiguous readings in anti-adoptionistic ways. This, apparently, is a problem for Ehrman's thesis, because he acknowledges that the scribes do not seem "to have been thoroughly consistent or rigorous in their attempts to rid the text of latent ambiguities and so to eliminate the possiblity [sic] of interpreting these texts in adoptionistic terms" (58). But Ehrman's thesis is precisely that scribes were motivated to clarify scripture, to make it say what it already meant. I, at least, find it unsatisfying that now he's willing to concede a lack of motivation in this direction. If a scribe didn't consistently change his text in a particular direction, then how can we with any confidence suppose that any specific change was meant to move the tradition in that direction?
Ironically, at this point where his thesis is the thinnest, I think Ehrman gets it the most right. Notice that he's conceding a point that undermines his larger thesis while trying to paint that point as congruent with his thesis.
[T]he majority of orthodox Christians, and presumably orthodox scribes, could live perfectly well with the text as originally written, interpreting it, that is, according to orthodox criteria and beliefs. Furthermore, the very process of transmitting texts was itself a radically conservative process. These scribes understood that they were conserving rather than creating tradition. (58)
Indeed. And however clearly Ehrman sees this, I think we can all accept that the reception of Ehrman's work—both among academics and in the popular media—has neglected this aspect of his work. That is, does anyone think of Ehrman as "that guy who thinks Christian scribes conserved rather than created the text of the NT"? Colbert could be forgiven for overlooking this aspect of his work.
1 Let me stress that I am dealing with Ehrman's argument up to this point (viz., p. 61); it may be that Ehrman addresses this issue at some later point. He does provide some response on p. 58, though I find his discussion there inadequate.
[Postscript: I also note the following humorous (but petty) "transcriptional" error. I appreciate how difficult it is to write and edit a book; it's impossible to spot and correct every mistake. Even so, I laughed out loud at Starbucks when I read the reference to "Revelations 2:2" on p. 36, n. 26. I'm sure Ehrman knows the actual name of John's ΑΠΟΚΑΛΥΨΙΣ, but it's especially funny since every academic hates this particular error.]