There are currently 2 USB powered mining devices that I know of. ASICMINER Block Erupter USB (https://bitcointalk.org/index.php?topic=195004.0) and     
Avalon Asic chip Mini USB miner (https://bitcointalk.org/index.php?topic=196281.msg2040708#msg2040708). The ASICMINER USB is currently in production, while the Avalon USB is still in design. Check out the Custom hardware forum (https://bitcointalk.org/index.php?board=76.0) to join group buys to purchase units. You should always read all the terms of the group buy before committing to an order.