Windows Genuine Advantage servers are down at the moment, meaning your Vista install will go into "reduced functionality mode" when you fail to check in with Big Brother, and your XP install will just be generally cranky until you can give that license information to The Man. Microsoft's tech support seems to be vaguely aware of the issue, and recommends trying again on Tuesday the 28th, since the server "might be down for a few days." Oh, is that all?
Update: Looks like they've fixed it. Thanks for the heads up, Marius.
[Via Boing Boing; thanks to everyone who sent this in]