Even the Steam version of the game will still require Ubisoft's proprietary Uplay client.
Update: An Ubisoft representative has confirmed that while the game will need Uplay, it is only for a once-off registration of the game, and any DLC you purchase. Once the game has been registered, you won't need to use the Uplay client.
Source: Lazy Gamer
Original Story: While earlier in the year, we learned that Ubisoft's South Park: The Stick of Truth would not require Uplay, Ubisoft's proprietary gaming client, i'm afraid the news isn't so good for fans of the publisher's flagship Watch Dogs. The company has confirmed to IGN that the PC version of the game will require Uplay.
"Like some of our other AAA releases, including Far Cry 3, Splinter Cell Blacklist, and Assassin's Creed IV Black Flag," said an Ubisoft representative, "Watch Dogs will be available on Steam and will leverage Uplay to support the games services." If it's anything like how Far Cry 3 worked, even if you purchase the game on Steam, you will still have to sign up and use the Uplay client to even launch the game.
It's a bit of a bother to be forced into using the client, but Uplay isn't quite as invasive as it could be. What is the most disappointing thing though, is that if you bought the game on Steam, you effectively have to have two game managers running, while the players who buy it from Uplay only need one.
Of course, this only effects the PC version of the game.
Previously, Ubisoft released the PC system requirements for the game, which recommended an 8-core CPU (uh-oh) and only 25 GB of free space (phew!)