Could just be a rarity, but not sure as I have yet to see a dungeon chest in 1.8. Nothing about them mentioned in the wiki about being found anywhere atm.
Could have sworn someone said they found one when breaking naturally generated pumpkins though.
The chests are still around. I've found plenty in the massive mineshaft in my world, and a few old dungeons connected to it.
I've only found melon seeds so far, though. My friend DID find a single pumpkin seed in a mineshaft chest (and damn, he created a perfect shelter for it to grow in and watched it to make sure nothing happened).