Powering from a non-dedicated power source
The Quick Start Guide says:
Micro USB power supply – make sure you use a good quality one, capable of providing at least 700mA at 5V. Do not attempt to power your Raspberry Pi by plugging it into a computer or a hub.
If I can find a powered USB hub that specifies it will provide at least 700mA, is there any reason why it shouldn't be used as the power supply?