Ubisoft have come up with a quite unique PC DRM system for their upcoming games Assassin's Creed 2 and The Settlers 7, in that you will now need to remain online to play a single-player game.
This means that should your Internet connection go out for any reason (maintenance by your ISP, squirrels chewing your wires, etc.) then your game will immediately jump back to the menu and you will be unable to play until your connection comes back up. Oh, and you'll also lose all progress since your last checkpoint (in the case of AC2 at least - in Settlers you will be returned to the exact point you left).
It also means that should Ubisoft's servers go down for any reason either, you will again be subject to the same blackouts until they decide to fix the issue.
I don't know about you, but my enthusiasm for this DRM is sandwiched right between having one of my testicles removed and being hit in the face with a hammer. And let's not forget, Ubisoft is the company that stated "as some of our games did not meet the required quality levels to achieve their full potential, they need more sales promotions than anticipated" in one of their quarterly sales reports; i.e. they don't mind pumping out substandard games as long as they can fluff them up with a lot of advertising to make you think they're amazing.
There is one silver lining to all of this DRM business and it is this: Ubisoft don't make any good PC games so this doesn't really change anything. I guess that's a little untrue, as the first Assassin's Creed was apparently ok and the sequel actually looks quite decent, but any interest I had in buying them has now evaporated thanks to this heavy-handed DRM. Seriously, even SecuROM wouldn't try to pull crap like this.
All I can say is to vote with your wallets and tell Ubisoft to stick it where Pluto is no longer a planet and not buy their games. Of course, you could pick up Assassin's Creed 2 on console where you won't have to put up with this nonsense, but you'll just be putting money in their pockets regardless.