Do you need to receive SMS? If you need to receive SMS, you will need
to host your own GSM device or modem so that people can send you SMS.
If not, you can just use internet SMS gateways like clickatell to do
the work, and post to them by HTTP, XML or email. The cost is about
6-8 cents per SMS. There are cheaper services, but not always
reliable. If you need to host your own GSM device, you can use
software like http://www.kannel.org (GPL Open Source) or http://visualtron.com/enterprise/sms-gateway.