Thankfully, a simple modification to Messenger's interface removes the tab. In this thread in the Neowin forums, Timothy (second post) provides a modified ContactWindow.pobj file that removes the useless tab. Download the file, and then replace the existing ContactWindow.pobj file. To do that, make sure Messenger isn't running, then control-click the application and choose Show Package Contents. Navigate into Contents -> Resources, and you'll find the original version of ContactWindow.pobj. Drag it somewhere safe as a backup, and put the modified file in its place.
Launch Messenger again, and the corporate tab will have been vanquished.
[robg adds: I tested this one, and it works as described (and the window definitely looks better without any tabs).]

