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.
The Phone Network protocol (PhoNet) is a packet-oriented communication protocol developped by Nokia for use with its modems. This is required for Maemo to use cellular data connectivity (if supported). It can also be used to control Nokia phones from a Linux computer, although AT commands may be easier to use.