It took a bit of tinkering but my install seems to have worked now.

If you already had songbird on your computer, MSC mode wont kick in as MTP wont let it install. I had to remove that addin and then browse and re-install MSC.

Following that, it prompted me to format and resync my music - i was a touch scared about doing so but jumped in anyway (i've got backups and can reflash if needed). It seemed to work all ok, all my music organised.

Playlists are currently missing and I read somewhere that will be fixed in an update of the MSC addin (or I did somit wrong!).

[system is windows 7 64bit and songbird 1.4.2]

Edit: I forgot the N900 would do its updating thing - after 5 or so mins all the playlists appeared so its sync seems to work good enough.