One of the items on my long list of blog todo's has been a change that the security team has been calling "FullTrust Means FullTrust" internally.  Basically, this change means that demands for identity permissions will now always succeed in FullTrust, where they may have failed in previous runtime versions.  Eugene beat me to the punch on this one, and has a good post up explaining the change and the reasoning behind it.  Definitely worth a read if you're interested in one of the bigger changes in security for Whidbey.