Apple said Monday that CEO Steve Jobs will deliver the keynote address at its Worldwide Developers Conference next month. Jobs missed last year's event after reportedly undergoing a liver transplant.

Apple in the past has frequently used WWDC as a platform for major product launches. The fact that Jobs is returning to the forum has sparked speculation that he might introduce a new version of the iPhone at the event, which starts June 7 in San Francisco.

It's widely rumored that Apple will launch a faster, 4G version of the iPhone, running iPhone OS 4.0, at some point this year.

Just weeks after an Apple engineer left one on a barstool in California, another iPhone 4G prototype was spotted earlier this month. Bloggers at the Vietnamese tech site Taoviet posted pictures of a device, labeled APL0398, that was similar to the phone lost in March by engineer Gray Powell.

Taoviet's photos showed a complete teardown of the alleged iPhone 4G, including shots of its internal, system-on-a-chip. Apple, for its part, has not confirmed the pictures' authenticity.

Apple also isn't providing specifics about Jobs' WWDC keynote, including whether he'll introduce new products. The company said only that the conference will feature five technology tracks, including Applications Frameworks, Internet & Web Graphics & Media, Developer Tools, and Core OS.

Apple said the conference is now sold out, with about 5,000 developers expected to attend.

In an indication that investors continue to link Apple's future prospects to Jobs' health, shares of the company were up 2.65%, to $248.75, in morning trading Monday on news that the CEO is returning to WWDC.