I have a NAS Server that shares my music. When I start iTunes 8.1 I can see the share but when I click on the share, it disappears. It never connects like it did in 8.0. When I restart iTunes, it shows back up again but never lets me connect the the share.
Very bad upgrade. I hope that time machine will let me revert back to 8.0 until this gets fixed.
You'll have to talk with your NAS vendor for a fix, most likely. They'll need to fix the iTunes/Bonjour server implementation and provide a firmware update to fix the problem. The information in this thread should allow their engineers to implement a fix.
I think that it is funny that the solution is to rely on your NAS Vendor for a fix when it was Apples iTunes 8.1 that broke the network share in the first place.