Grabbed the souce and had a quick look around. There are two drivers at the moment: phonet and atmodem.

Phonet is just a skeleton with no useful code at the moment, but its presence confirms that it is/will be relevant to maemo. Atmodem is more fleshed out and contains some explicit checks for TI Calypso chips which suggests that (somewhat ironically) a Neo1973 or Freerunner might make a good ofono hacking platform.

BTW, tt's not documented in README/HACKING, but in order to run ofonod you'll need to modify the system dbus security policy to allow it to connect (see src/ofono.conf).