During his recent debate with Richard Bauckham, Bart Ehrman argued that the documents Papias refers to as having been authored by Matthew and Mark were different than the gospels we have today. He also suggested that Justin Martyr may have attributed the gospels to authors other than the traditional ones and that the Gospel Of Peter was part of Justin's collection of gospels. I've responded to such arguments in the past. See my post at 11:18 P.M. on 9/19/06 in the comments section of the thread here. My comments there include a discussion of the evidence that Papias addressed John's gospel and its authorship, not just Matthew and Mark. On Justin's alleged acceptance of the Gospel Of Peter, see Lydia McGrew's comments on the subject in a response she wrote to Ehrman last year.
The more Ehrman proposes that sources prior to Irenaeus had a different collection of gospels, the worse of an explanation he's providing for why there's such widespread agreement about the four canonical gospels from the time of Irenaeus onward. Why do the different gospel collections that allegedly were accepted earlier leave no explicit trace in the historical record and so few allegedly implicit traces?