These are the first screenshots of Tomb Raider Underworld on the DS, and it looks quite good.
By the looks of things, the game retains its 3D polygons on the handheld (with fairly detailed textures, we reckon) but takes on a 2D gameplay viewpoint, which should keep things tight on the simpler D-pad controls.
There also appears to be mini games, including a Tetris-style puzzle, played on the touch screen.