I’ve seen the same thing. That even if you tell iTunes to put the database somewhere on the network, it keeps the .xml file locally. Lame.
One (subpar) way around it might be to copy your itunes .xml up to the server on logon (or logoff — win2k or better has shutdown scripts) to your machine. mt-daapd should see the updated .xml file and import it next pass.
I don’t know if that solves any problems, but it’s something that might help in some way. :-/