Glass vs. PET bottles on long term (1 yr.) storage.

I started brewing (again) about 18+ months ago. I recently tried 2 brews that I'd brewed awhile ago. A "Fire in the Hole" chile beer, bottled 11/16/10 and tonight, an AHS Winter Warmer that I bottled, 1/18/11. Both were kinda flat, but the WW had deteriorated in to a sour/bitter mess. The WW had coriander nutmeg and mace for the winter spices.
Austin Homebrew Supply
Is it just that PET bottles aren't any good for storage over 8 months? Other than that I love using the PET bottles.

they must not be. I only ever use glass but it sounds like the pet bottles aren't as airtight as we may think

An addition to this, I tried a Guinness Foreign Extra Stout clone I brewed last St. Paddy's day. Also flat....

I use PET bottles all the time, but never long term. Plastic containers breath. I guess would be a good term. To bad about the brew, Maybe try to re prime? ?????? I wonder if that would work.