For our purposes Kannel is an SMS gateway that can easily be configured to talk to RapidSMS over HTTP. Personally I've found it to be more reliable than PyGSM for connecting directly to a modem [1], but your mileage may vary. It also provides seamless integration with SMPP gateways that can sometimes be obtained from telcos. There's a "kannel" package in Ubuntu that you can install to get it running, and you can find out more on the Kannel website [2]. The gist I posted [3] shows both the Kannel configuration and RapidSMS settings necessary to get it working.
Somehow it doesn't look like we have a How To in the docs on setting up RapidSMS with Kannel yet, so I'll put something together in the next few days and post it to our lovely new documentation site on readthedocs.
Cheers,
Tobias