For DIY ... you can get a small ARM board with sim slot with linux installed.
Buy a sim card or make a contract, like for regular phone.
Using opensource software such as
Gammu you can program your app to send sms via above little box.
Quote:
The Gammu package does not include just this binary, but as well Gammu SMS Daemon, Gammu library and Python bindings which you can use to develop own application accessing mobile phone.
Of course, you can always get a sms gateway service online, as suggested.
Hope that helps
Regards
Peasant.