I too am having this problem. I have several portals setup, some of which work correctly when adding or hiding items in global navigation, and some which don't. The two that don't work were both setup initially with the Report Center template. Is there something special about the Report Center template that would affect the global navigation options
The exact behavior is to go to the Site Navigation Settings page. It doesn't seem to matter what is checked for the Subsites and Pages or Sorting. Under Global Navigation if I add a heading and click OK there are no errors but the page does not add either and going back to the Site Navigation Settings page, it's like nothing at all happened. This is the same if there is an existing page and I want to make it hidden. I can hide the page successfully if it appears under Current Navigation, and appears like it works under Global Navigation until you click OK, then it just reverts to the way it was. There are no database errors, no application log errors, no error from SharePoint. It just appears to do nothing.