I just had search, then go through all this...
Okay...since Blizzard is totally ignoring everyone, I will post what I did to get it working.
It's a combination of several things that folks have tried.
1) Delete the following folders:
c:\program files\starcraft ii\battle.net
C:\Program Files\StarCraft II\Mods\Core.SC2Mod
C:\Program Files\StarCraft II\Mods\LibertyMulti.SC2Mod
also delete this file:
C:\Program Files\StarCraft II\Mods\Challenges.SC2Mod
IMPORTANT: delete the files, but leave them in the Recycle Bin (DON'T EMPTY THE RECYCLE BIN!).
Restart the StarCraft 2 program and let it redownload the patch. If you try to run it, it will probably give you the message "files are missing from your StarCraft 2 installation".
Go to your recycle bin and restore the folders "Core.SC2Mod" and "LibertyMulti.SC2Mod". When you are prompted if you want to merge the folder you're restoring, answer "yes", but make sure to answer the questions about replacing files with "NO". This will restore all the files that were not updated (the missing files).
Now when you run the StarCraft 2 program, it should take you straight to the "play" option. When you log in to Battle.Net, expect delays as it's downloading more files. This took about five minutes on my system, but once that was done, the game started just fine.
You're welcome, Blizzard.
...just to get a game of StarCraft 2.
You can keep your "open" system, Newell, et al.
"Closed" systems where things work like they're supposed to, FTW!