I have the same problem
I am using Uno r3 and GSM shield from open-electronics (http://www.open-electronics.org/arduino-gsm-shield/)
I did un comment #define GSM_TXPIN 5 and #define GSM_RXPIN 4 in GMS.cpp.
Trying to run "GSM_GPRSLibrary_Client.ino" from SIM900IDE100