I recently dug a black plastic flip-close food container that was marked "compostable" out of my parents compost pile, after burying it there almost exactly four years ago.

It looked exactly the same as when I put it in there. Interesting experiment but that's what I was already expecting, based on the very small amount I'd read suggesting that one needs to put plastics marked "compostable" into some kind of weird high-temperature bioreactor in order to get them to break down. nkurz's article seems to delve deeper into this topic.