On Basic Brewing Radio, they recently had a show where they did a side-by-side tasting of an all-grain beer brewed with 10 lbs of pumpkin, and one that was an extract-only beer brewed with no pumpkin. Most of the tasters said that the beer that had NO pumpkin actually tasted more like pumpkin.
Pumpkin will actually ferment out into the beer, leaving VERY little flavor behind. That being said, if I ever make a pumpkin ale I would probably throw in a little pumpkin just for the authenticity. But I would do so knowing that it would have very little effect on the final product, if any.