This is Tim Schafer's new game (if you don't know that name, shame on you, but I'll help out anyway: Monkey Island, Psychonauts, Brütal Legend, Grim Fandango...). It's about Russian Babushka dolls (the ones that fit inside each other). You play a particularly tiny doll, and you spend the game solving puzzles by hopping inside bigger dolls and taking control of them. The various dolls have unique abilities, running the gamut of pointless to functional.
The puzzles aren't difficult (at least not so far, but I'm only on the third level), but the gameplay overall is fantastic. It's such a simple game, but it's so well-realized that it feels more robust than most "full size" games. The world is thoughtfully and consistently represented (the scale is obviously suited to the size of the dolls, so you see things like plastic cocktail swords appearing as "real" items), the controls and functionality is consistent and supports exploration and just having fun and, best of all, the game's sense of humor is fantastic.
It's on PSN now (probably on XBLA as well, but I haven't turned my Xbox on in months so I don't know for sure). It's excellent.
Some random screenies courtesy of google image search:
Static pictures don't portray the vibrance of the world, though...the animations, the sounds, the artistic style just doesn't come through in stills.