We'd expect at least to see a new version of iOS, iOS 6 unveiled. Whether or not we see an iPhone 5 introduced remains unknown.
There have been rumors that Apple is attempting to return to a mid-summer release after last year's October iPhone 4S launch. Naturally there are also rumors running counter to that, saying Apple will keep to an "at least" 12 month schedule, for various reasons, including not wanting to anger those who recently bought an iPhone 4S.
Last year, the event sold out in less than 12 hours, so we would recommend those wanting to buy tickets do so immediately. You can do so by going to the Apple Worldwide Developers Conference 2012 website (developer.apple.com/wwdc). The $1599 tickets are available at Apple's developer website, with purchases limited to one per person and five per organization.
Those who are unable to attend WWDC 2012 will be able to access and watch all of the technical sessions from the Apple Developer website (developer.apple.com/videos) after they are posted, for free.