Most folk I know just use a cell phone to do this. Your post makes me think you just started on this project and that's OK but be sure to do some more research.
I'd like a low-priced (between $20-$60) GSM modem that I can send SMS through using the linux tool SMS server tools 3. It needs to be able to accept standard AT commands. I have an AT&T SIM card, so unlocked or AT&T enabled would be good. I don't want one that just works with its own software, I need to be able to interface it with other programs.