Did you use allspice? I had problems carbonating my pumpkin ale last year and the first question from my LHBS guy was if I used all spice. Apparently there are oils in it that make it take a lot longer to carbonate. I brewed in September 07 and wasn’t carbed very well in February 08. I opened one up on a cool day in August 08 and it had a lot of carbonation with very little temperature change where I was conditioning.
One solution, just call it a Pumpkin Mild and the carbonation level will be perfect.