I've had some success giving the fixed cup tool a few gentle taps with a hammer. Clamping the tool in place, as [MENTION=21724]cyccommute[/MENTION] shows, helps. Penetrating oil helps. Presence of rust suggests that the cup might be in there pretty good.

If you can clean the cup while it's in place, and see that it's not pitted, then it doesn't really have to come out. You could still put in a couple drops of penetrating oil, and then leave it for next time.