there are a number of ways to do this:
1. use an existing service (SMS gateway provider) which will charge a norminal fee for each message. fast, easy and reliable - they provide all the api calls you need and can even do email to sms so all you do is send an email in certain format to a predesignated address and you are done.
2. hooking up a phone to your usb/serial port, find a phone that is compatible and load the driver for the phone, then program it to to connect to your phone's sms service center to send the message. you still need to pay for the sms message through your wireless provider. (do a Live search for SMS Service Center, Encoding PDU message)
you'll need to pay $$$ either way so it's best to go with the first option unless you plan to send a ton of SMS's
