It’s important to differentiate between the iTunes library files and the iTunes media folder. When I did this I didn’t put my iTunes library on the network-attached storage, only the iTunes media folder. This is how I did it.
Copy your iTunes media folder over to the FS using Finder on the Mac or Explorer on Windows. Back up your iTunes Library files (~/Music/iTunes/ on the Mac, no idea on Windows). Shut down iTunes.
You are now going to make a new empty iTunes Library, then add all the stuff from your FS to that. Use this Apple article to create a new iTunes library: http://support.apple.com/kb/HT1589 and put it somewhere on your local disk.
When you have that done, go into iTunes preferences in your new empty iTunes and change your iTunes media folder location to the new folder on the FS. The preference is under “Advanced” on the Mac. Don’t choose the option(s) that would do anything to your files. Also in the Advanced tab, deselect “Copy files to iTunes media folder when adding to library” for the moment (you can select it again afterwards if that’s how you normally run).
Go to the File menu, choose “Add to Library”, then navigate to the top level of the iTunes folder on your FS, and “Open” it (on the Mac at least). Sit back while iTunes adds all of your files to your new library.
The only downside to this is that you will lose your ratings, play counts, etc. Artwork should come across intact.
When this is done, you will end up with your iTunes library files on your local disc, where they are nice and fast to access, but all your media will be on the FS. I’m doing almost the same setup myself, except I have my media on a Firewire attached 4-bay Drobo.