When I made my pumpkin porter, I used the AHS London Porter extract kit. I twice baked ~60 oz of pumpkin and then added it to the boil for all 60minutes. I can't say for sure that the pumpkin helped bring out the pumpkin flavor, but I did use a fair amount of pumpkin pie spice, ginger, and nutmeg at flameout. The beer turned out wonderful.
My wife did this for her first brew last fall, it turned out awesome and was a really easy recipe. The book is full of extract clones of a bunch of popular beers. I do A/G but enjoy doing these every once and a while. Worth the 5$ I paid on amazon for it at the time.