This week I uploaded QTScrobbler to Debian.
If you have a music player running Rockbox, you can configure it to write a log of what you have listened to, and when, to a
.scrobbler.log file in the root of your player's file system.
QTScrobbler takes such a log file and submits the contents to the last.fm audio-scrobbler website. It then deletes the log file from the player.
If that sounds useful to you, give it a go!