Retailers mention a Steam account is needed, alongside Xbox Live and PSN for consoles, but Uplay is absent. It seems retail copies are by default Steamworks versions.
Ubisoft does of course intend to sell Watch Dogs through their Uplay store but it's unknown at this stage if Uplay will be needed at all in order to play the game.
Ubisoft's PC titles through Steam mostly end up opening the Uplay launcher where you then select to play the game, making the Uplay platform a mandatory checkpoint. It's of course possible that the same will be required of Watch Dogs but retail versions do appear to activate with Steam, which is likely an additional selling point.
UK retailer GAME mentions Steam in their Pre-Order Terms and Conditions at the bottom of. Amazon's listing makes no mention of either Uplay or Steam. Watch Dogs supports drop in/out multiplayer.
Watch Doges releases on PC, Xbox 360, PS3, Xbox One and PS4 May 27th.