Apple chief executive Steve Jobs on Wednesday was not listed as among the executives who would participate in the opening keynote at the company's Worldwide Developers Conference.

Kicking off the San Francisco event will be a team of execs led by Philip Schiller, Apple's senior VP of worldwide product marketing. The WWDC is scheduled to offer technical sessions on Apple's latest version of the iPhone operating system and on the upcoming Snow Leopard, the name of the next version of Mac OS X.

Jobs is on a six-month medical leave because of complications stemming from an operation several years ago for pancreatic cancer. The possibility of the tech industry icon and Apple co-founder attending the WWDC was slim, given recent comments by Apple CFO Peter Oppenheimer.

During a teleconference with financial analysts last month, Oppenheimer said, "We look forward to Steve's return to Apple at the end of June." With the WWDC scheduled for June 8, the comment made it unlikely Jobs would make an appearance.

But while not getting the chance to see Jobs, developers have been promised a final preview release of Snow Leopard, "so they can see the incredible progress we've made," Bertrand Serlet, Apple's senior VP of software engineering, said in a statement.

Tech sessions on the Mac OS will showcase hundreds of refinements to the operating system and take a "deep dive" into new technologies, including a 64-bit architecture, QuickTime X, and multicore and GPU processor support, Apple said.

When Snow Leopard hits the market, it will be going head to head with Microsoft's Windows 7, which could be completed as early as August.

On the iPhone OS, the sessions will cover the more than 1,000 new application programming interfaces available for the OS, along with Apple's software development kit.

In general, the WWDC will offer more than 100 tech session by Apple engineers on a wide range of topics, according to Apple. There also will be hands-on labs giving developers the chance to work one on one with engineers.

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