The Raspberry Pi Wiki entry on power suggests this is possible (emphasis mine):
Back-Powering; (powering the Raspberry Pi from a USB hub through the uplink/data port, single cable) Back powering is possible on the Raspberry Pi. Revision 1.0 boards have to be modified to back power, this is due to the 140ma "polyfuses" that are installed in the USB port circuit. Revision 1.1 boards do not need modifications to back-power, they have replaced the polyfuses with 0ohm resistors in their place. Revision 2.0 boards do not need modification, they have neither resistors nor polyfuses. It is advised that short (12" (.3 meter) or less) USB cables be used for back-powering a Raspberry Pi. Cable resistance plus connector resistance can quickly reduce operating voltages below the proper range(5.25V to 4.75V).
but also by back-powering it, you are actually bypassing the PI's input polyfuse protection device! This can have extreme consequences if ever you manage to put more than 6V on the PI, even for a very short period. As this causes the overvoltage device D17 on the the PI to trigger and short the 5V supply! Without the polyfuse limiting the current through D17, it will burn out, probably melting the PI's enclosure with it, (if you have any) and possibly causing a fire-hazard. It will probably also create a permanent short of the 5V supply! So be warned, and if you use back power make sure your hub or its PSU has a fuse to prevent this from happening. If not, add your own fuse.