Casual games used to fall into three camps: the Bejeweled clone, the Diner Dash clone, and the Mystery Case File clone. It was and still is a clone-based market, and few are better at producing successful incarnations than PopCap. Its version of the Mystery Case File games began with Mystery P.I. - a hidden object game in which you're tasked with finding particular objects in an extremely cluttered scene. That sort of thing seems perfect for the DS, and it's surprising there haven't been hundreds already. Now PopCap has released two at once.
Mystery P.I. Portrait of a Thief
PopCap has previously done a couple of Mystery P.I. games on PC, The Lottery Ticket and The Vegas Heist, and Portrait of a Thief borrows very heavily from the former. Indeed, many of the same screens are used, this time to "tell the story" of an art theft that you're "investigating".