Yesterday we saw reports of people modifying files on their Xbox 360 hard drives to get at some wild abilities in Left 4 Dead. But Valve responded today with a server-side solution that will, at least, lock down dedicated servers and keep them safe from nefarious harm.
User-hosted games, system link matches, and versus mode, however, are still on and popping. If you're still feeling the sweet, sweet grief of infinite molotov cocktails or... big giant versions of Bill or something, you'll have to wait for a patch, which will be released in "the coming days." And here you thought you didn't have anything else to look forward to in 2008!