Sony Ericsson Walkman phones like the W710i or W800i can also be used with this plugin. You should connect the phone via USB, and put it in "File Transfer" mode. Make sure you change Song Location in the configuration to /Music/%artist/%album/%title.%filetype, or the player won't see them (be careful if your device is mounted by UTF-8 encoding, it will be case-sensitive and you must configure the song location to /music/%artist/%album/%title.%filetype). Disconnecting the phone (i.e., running eject) might take a minute or two, as the system finishes copying in the background; you should let it finish. Using Amarok to transfer music is a much better experience than the primitive (and slow!) Disc2Phone software that Sony Ericsson provides.
The W710i supports mp3, aac, m4a and wav files.
Unlike a regular music player, a music capable phone has other kinds of data in its memory, so it's quite likely that you'll need to put the files in a special location, like on the Walkman phones.
