I recently converted several simple WordPress installs into a single WPMU install. As I use all of the blogs myself I wanted a quick way to switch between the various backends. This is probably only useful if you don't have too many blogs and use all of them yourself. The switch links will be added to your super admin menu.
By the way, I used to use the Ozh admin menu plugin. You might need two clicks to switch if you don't.
Update: This plugin is probably obsolete with the new admin bar in 3.1
WordPress plugin demos are disabled as I don't use WordPress on this site any more.