With Twitter what we did when we launched originally was simply get a PCMCIA GSM card and used some windows software to post it to the website via a protected REST api. Now i guess you could use a usb gsm modem. Worked great for prototypes. Now i guess there are services, but really the usb gsm modem method works well even today.

You can't charge for SMS that way and technically you're supposed to use a short code for automated sms sending. Short codes are very expensive, so don't do that til much later.