Ubisoft announced on its website that certain game DRM-enabled game would become temporarily unplayable on February 7th while the company transferred some of its hosting services from one third-party facility to another. The notice does not specify how long the outage will last.
Most of the games effected are older titles, so hopefully the outage won't ruin too many players' days. Here's the full list of games going dark:
ASSASSIN'S CREED - Mac
Tom Clancy’s H.A.W.X. 2 - PC
Might & Magic : Heroes VI - PC
Splinter Cell Conviction - Mac
The Settlers 7: Paths to a Kingdom - PC
The Settlers - Mac
On the other hand, Ubisoft went out of its way to point out that many of its current titles shouldn't be effected by the change, including: Assassin's Creed: Revelations (PC, PS3, Xbox 360, OnLive), Driver: San Francisco (PC, PS3, Xbox 360, Online), Just Dance 3 (Xbox 360), The Settlers Online, and Anno 2070.
UPlay support, however, will be shut down for all games, but during the shutdown PC games will be allowed to operate without it. If you've just bought a new Ubisoft game, I'd make sure to play it at least a little right now, because with UPlay off, new copies of PC games can't be authenticated and therefore won't run until after the transition is complete.