IIRC, the Seeeedstudio shield is as beefy as it is re: capacitors (470uF vs. OEM recommended 100uF) for precisely that reason, i.e. to allow the shield to function even if it's 'just' using USB power. The maximum power draw is around 2A, though only in very short bursts. Hence, the big capacitors.
The Seeedstudio shield has worked fine for me in a GPRS application, using a PC as well as a Mac USB port as a power source, though I realize that not all USB ports are created equal (i.e. some allow more than 500mA to be drawn). Similarly, a GPRS / Arduino board I built is happy with just USB power. Both the Seeeedstudio as well as my board use the Simcom 900 chipset.