Works for me (though I only could test it with the native Gstreamer backend).
It writes ID3 v2.3 via taglib#. Note that the following ID3 tags that were generated using Gstreamer are _missing_ when using this patch:
TSSE ("Software/Hardware and settings used for encoding")
TXXX[MusicBrainz DiscID] TXXX[CDDB DiscID]
The first one is just "Banshee <banshee version>". For the other two I couldn't figure out where these tags are set.
