Hi there!
I am on that same path! I tried the thumbnails synchronization with symlinking the thumbnails cache on a network share to share thumbnails from win 7 to xbmcbuntu, it worked perfectly!
Thanks to hernandito for that great tutorial!
What I have found though is that it is a little slow on my client machines and even on my win 7 pc which holds the mysql database. I think it is due to the thumbnails cache being on the network share and not locally.
So I searching through the boards to find an automatic synching system that syncs my local thumbnails cache from my "master machine" to a network share and from the network share to the clients. What I have seen rsync and cwrsync seem to be a good solution. Though I didn't have the time to try it yet. Did you guys make any progress on this? Especially the sync from xbmcbuntu to a win 7 machine?