During the five-day event Apple will "unveil the future of iOS and Mac OS,” said Apple's VP of Worldwide Product Marketing, Philip Schiller.
In less vague terms, this probably means that Apple will unveil a new version of iOS (recent rumors claim it will be a total overhaul of the platform) and Mac OS X Lion, which is due for a Q2 release. Furthermore, Apple launched the iPhone 4 at last year's WWDC, and the iPhone 3GS as well as iOS 3.0 at WWDC 2009, so it's likely that this year's conference will also bring us the next generation iPhone.
For attendees, the WWDC 2011 includes more than 100 technical sessions presented by Apple engineers (more than 1,000 of which will be there to help developers make the most out of iOS) and the Apple Design Awards for the best iPad, iPhone and Mac apps.