Installed latest Global Search plug in from WPMU, Activated, and used the back end style features. The page was generated and the search form was on the page. However, when I search for known words in various blogs in the network, including the local one, there are never results. I search on a period or blank line, same results.
If it is based on the plug in by aliciagh, if there is a clash with 3.1 I am sure she'd like a heads up on the fix.