MacRumors suggests the port could actually be for Light Peak.
Light Peak prototypes have been using hybrid USB 3 connector connectors that might easily be confused for USB. The exact connector for Light Peak hasn't been described, but it may be close enough to USB to be a case of mistaken identity in the early iPad 2 rumors. The opening for the port may be a bit small for a full-sized USB connector, though it seems feasible that the Light Peak could also adopt a mini USB style form factor as well.
Interestingly, CNET reports that Apple is expected to make an announcement about a new high speed connection technology