Hack prevents public playing on Battle.net servers
If you're trying to play a UMS game on an official Battle.net server these days it may happen that you hit an unusual brick wall: There are no games to join. Somebody made a program that removes every UMS game created from the list and is currently being abused to affect every Battle.net server. Private games that are password protected are not affected by this attack.
There seems to be no response from Blizzard and rumors claim a solution in the coming days is rather unlikely. This attack does not seem to be affecting non-official StarCraft servers such as iCCup, Fish or Brain.