Look at the schematic: http://www.sparkfun.com/datasheets/DevTools/Arduino/cellular%20shield-v12.pdf
It uses Vin, not 5V, and has an onboard regulator. I'm surprised it works if the Arduino is running of USB power instead of Vin.
It also has the option to use D2 and D3 instead of RX (D1) and TX (D0).