It's an intriguing concept, but from what I've heard, the whole movie is Reynolds in the box, which is interesting, but at the same time I feel like that could get really old really quick. Still, Reynolds is a very capable actor and it's cool to see him do something that actually seems watchable (Paper Man not withstanding).
Buried comes out September 24, 2010, maybe in limited, maybe not. Considering September is a boring movie month, I wouldn't be surprised if its distributor didn't just say "screw it" and put it in wide to roll the dice.