As you see, the logo’s simply a “7” filled with a variety of colors, just like the WWDC logo. It has been speculated that each color in the WWDC 2013 logo represents a redesigned stock iOS app. The tagline for this year’s WWDC is “Where a whole new world’s developing.”
If you like the look of the logo, and want to set it as your wallpaper, Surenix has created one for your iOS device.
Update: A banner featuring an OS X logo has gone up at Moscone, with a backdrop of a wave. Sea lion?