I've mentioned previously on here my suspicion that the Doubting Thomas pericope is interpolated. I'm heavily indebted to Gregory J. Riley's Resurrection Reconsidered here. Unfortunately, I lost my copy somewhere or other some time ago, so can't provide specific references. I'll update at some later date with those in mind. Without further ado, Four Reasons to Doubt the Authenticity of the Doubting Thomas Pericope
1. There is no hint in the preceding pericope that Thomas is not present. Not only that, the qualifier that begins the pericope (20.24) that "Thomas, one of the twelve, called the Twin, was not with them when Jesus came," betrays an author's awareness that there is no such hint. It makes the most sense to view this as a qualifier to an existing section, not as part of a flowing narrative.
2. 20.23 moves smoothly into 20.31. All of the promises have been fulfilled, the commission has been given. The DT pericope sits oddly in the midst of that.
3. It's redundant. Nothing is said of whether or not Thomas actually touches Jesus (though the tradition that he did is very early indeed), and the conclusion has nothing to do with touching--"You have seen and believed." But why is the DT pericope necessary to that end? Couldn't "You have seen. . .blessed are those who have not. . ." have flown from 20.20? The other disciples needed to see, the same as Thomas did.
4. Following from point 3, the implication that Thomas isn't blessed (like those who have not seen and believed) from 20.29 makes no sense in the light of 20.20. Surely the other disciples are blessed, yet they needed to see to believe as well.
For posterity's sake, a pair of arguments that it's authentic:
1. Thomas has three dialogues, all of which pertain to the resurrection (11.16, 14.5, 20.24-29)
2. 20.28 fulfills 5.23 "all may honor the Son, even as they honor the Father"