Normally, when a game is updated and things break, it's cause for concern. When a Valve game updates and things get weird, though, people get excited.
Ever since Portal 2 was announced via updates to Portal 1, people have paid extra close attention to any changes made to Valve games, and after a big one to Half-Life 2 this morning, those people are going a little crazy.
A patch sent out over Steam earlier today - for some people hundreds of MB in size, for others over 1GB (sometimes even over 3GB) - has made some weird alterations to the game. Many people's versions, for example, now feature Korean audio, even though their game settings specify English. Like mine. I can't even change the language, it's stuck on Korean. Some people are also reporting broken save game files.
It's probably an error, and we've contacted Valve to be sure. But when it comes to Valve games, there's always hope stuff like this is something more, even if it is a fool's hope...
UPDATE - While we're yet to hear back from Valve, the game, like other Source Engine titles, was recently converted to the "Steampipe" file system, so it's likely an error with how that handles your update downloads.
UPDATE 2 - Valve tells us it "should be fixed now".