When I'm listening to radio streams through Banshee, it doesn't scrobble the songs to Last.fm even though the artist and title come through correctly. 

I had a quick look at the code and it looks like you need to add the PlayerEvent.TrackInfoUpdated event into the OnPlayerEvent method in Banshee.Lastfm.Audioscrobbler.AudioscrobblerService (although I couldn't get the code building on my machine, so couldn't test this). The AppIndicator extension (located in the Community Extensions) correctly displays a notification when the song changes, so it's definitely possible to detect.

Steps to replicate:
1. Ensure you have a Last.fm account set up in Banshee
2. Listen to a radio stream (eg, any AACplus stream on http://www.di.fm/)
   => Expected: Song gets scrobbled to Last.fm
      Actual: Song is not scrobbled to Last.fm
3. Wait for the song to change
   => As above

Environment:
 - Ubuntu 11.10
 - Banshee 2.2.1 installed via Ubuntu package