Kannel is a great platform for all things mobile. We use the SMS gateway feature with our service management system to notify customers when their machines are ready for pickup. All you basically need is a mobile phone which you can connect to your Mac via USB and Kannel.
Surprisingly, it's not available through MacPorts or Fink and compiling it from source has been rather tricky in the past. Through some experimentation I found that the CVS version works better on 10.6 and was even able to build a universal binary of it. We've been running this version for months now on a Mac mini and an old Nokia phone.
You can download the installer from here. In addition to the application it also includes launchd jobs for bearerbox and smsbox. Check /etc/kannel.conf before loading them in. The full Kannel documentation is available here.
I've only been able to test this with 10.6, let me know if you have problems on 10.5. One day I hope to find time to write a more detailed article on how to set this up…