By pressing the middle PS button on a PS3 controller, the DualShock3's rumble can be toggled on or off from the Controller Setting menu in-game. Listed as the "vibration function," rumble will obviously only be felt with DualShock3 controllers and not the rumble-less Sixaxis.
Sony plans to release the DualShock3 in Japan this November, with the peripheral coming to the U.S. and Europe this spring. Until then, check out the Shack's list of game's that will feature or be updated to feature DualShock3 rumble support.
Speaking of Ratchet & Clank Future, prospective owners should be sure to check out the minor and easily resolved bug that prevents some from installing the game.