Last month Mark Goodacre posted a very brief response to recent discussions amongst bloggers on the synoptic problem. I link to it here only because the erosion in the confidence with which NT critics hold to the Q theory is fascinating. Not simply that many scholars no longer "believe in" Q (the theological-esque language is interesting here), but that even many of those who do support the Q hypothesis do so more tepidly.
Goodacre's post is well worth reading, and the links worth following. Note also the article by E. P. Sanders referenced in the comments section.