In reply to:

If an ID3 tag gets changed, only the tag is re-transmitted, not the whole MP3 file. Although this probably is not true when you change ID3v2 tags, which are inserted at the beginning of the file.

Rsync should be able to deal with changes (including additions and deletions) at the beginning of a file just as well as at the end. So I don't forsee problems with that.
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Toby Speight
030103016 (80GB Mk2a, blue)
030102806 (0GB Mk2a, blue)