As of April, 2010 it is very difficult, if not impossible, to find a ranked match. You will usually get a "match no longer exists" message when it attempts to join.
Midway Games (publisher of UMK3) has gone bankrupt and their assets were sold to Warner. Normally, games hosted by Microsoft (pretty much any non-EA game) will continue to work on Live forever, though in this case that seems to not be the case. Maybe one of the Live updates broke it and there's nobody at Midway to fix it.
Hopefully, whoever has Midway's assets will restart the studio and get someone to fix the game.