Hope this helps someone else. I just figured out how to get Itunes 10 Home sharing to work with NIS 2011: Open NIS 2011, then under "Network Protection" heading, click on "Network Security Map." Locate the computer you're trying to Home Share with, and click on its name. Then when the details for that computer appear, locate "Trust Level" and click on "Edit" to the right of it. Put the trust level at "Full trust." Do this on both computers, each "Full trust"-ing the other. That solved my problems with Home Sharing on Itunes 10.
-Matt