Well, it seems the rumour mill has a decent bit of fuel added to it. Watch Dogs appears to have broken cover a little early. The screenshot below cropped up on Reddit showing what seems to be an accidental early release of a Closed Beta client for Watch Dogs. This certainly seems like Ubisoft are getting towards a version of the game they're happy with now and are looking to open the game up for some feedback soon.
Considering the file is quoted at a touch over 23GB, I'd say this seems like the real deal (as opposed to a demo or a mislabelled wallpaper). It also seems that this is a more fully fleshed out beta allowing both use of the single player mode (perhaps a select chunk of storyline) along with the multiplayer mode where you attempt to cause merry mayhem upon hapless players before cutting a swift getaway.
The description alongside the game also gives a little insight for you as well, which I'll post below for your perusal if the screenshot proves a little too blurry for you.
You are Aiden Pearce, a brilliant hacker but also a former thug, who's criminal past lead to a violent family tragedy. In a world led by technology, you will be able to hack and manipulate the city's systems to stop traffic lights, detonate gas lines, turn off the electric grid and more.
The City of Chicago has become the ultimate weapon for a man bent on revenge.
If you don't quite make it into the beta whenever it pops up, you'll be able to grab it on Xbox One or Xbox 360 on May 27th.