For those unclear with how the multiplayer works:
According to Ubiblog, you are always Aiden Pierce, the main character, however you will appear to be a simple NPC to other human players if you jump into their game.
In Watch Dogs, when you enter someone elses game, you will appear to the other player as not-Aiden. In other words, if I hack into your game, I will see you (the hackee) as yet another NPC-esque character on my screen. And if you hack into my game, you will continue to be Aiden on your screen, but I will see you, likewise, as a generic in-game character.
Not only that but it also seems there is an unrestricted choice of what you can do once you have jumped into someone elses game, examples such as ramming into another player to stop them from getting to a mission or simply being a voyeur were given.
To prevent any interruption when doing main and side missions, the multiplayer gets turned off automatically to prevent the griefers from getting in your way, however when you are not in main or side missions the multiplayer is turned back on. There is an option to remove multiplayer overall however thats no fun.
Finally there will also be multilayer missions in which you may have to work or work for another player.
Colin also assures us there are plenty of multiplayer missions in which youre tasked with protecting another player, so its not just about adversarial actions.
Watch Dogs currently has a release date of Holiday 2013.