the Raspberry Pi is NOT meant to be powered by a USB port!
It may have a mini?/micro(?) USB power jack but every bit of nooby documentation says to not try using a computers USB port for power.
Fortunately for me, my girlfriend is an iFashion victim and breaks iThings regularly so there are heaps of 500 and 750 mAh wallwort USB charger things here. (Telstra have dibs on her soul)
The new Raspberry Pi has a "decent" power supply onboard instead of the linear regulators of the early models.