Yes, I did eventually figure this out. A big part of the problem was that I was using AT+SSTRSEND instead of AT+SDATATSEND. Only AT+SDATATSEND can support the line breaks necessary for an HTTP request. I've put up my code at GitHub - tobek/SM5100B-GPRS: Arduino program for sending HTTP requests over GPRS using the SM5100B GSM module. It's possible. - the readme shows all of the commands I send and the content of the packet, and there's an Arduino file that does it all. Good luck!