Apple sent out an email and is advertising on their Developer Connection page inviting registrations for the next World Wide Developers Conference at Moscone West conference center in San Francisco California. The dates are June 8th through June 12th. WWDC has historically been an event in which Apple (and usually Jobs himself) has revealed big software announcements, since the focus of WWDC is software rather than hardware.
The page and email feature an iPhone with the words "Make Your Mark Here " (or if you watch the page for a bit, "Make This Your Year" slides over as if you were switching between home pages) spelled out in the place of apps on the iPhone. This suggests that the focus of the event will be on developers and the new tools featured in the iPhone 3.0 SDK.
There also might be the chance of an announcement for the release date of OS X 10.6 "Snow Leopard" which has seen multiple new seeds to developers in the last few weeks. It would not be a big surprise, as Jobs confirmed the name "Snow Leopard" and announced the iPhone 3G at the last WWDC in 2008.
This years WWDC will feature sessions on iPhone, and Mac OSX Snow Leopard as well as OS X Server. Registration for WWDC 2009 is $1295 before April 24th, after which the price will jump to $1595.