Apple Inc Chief Executive Steve Jobs returns on Monday to the stage at San Francisco's Moscone center to take the wraps off what investors hope will be the next source of growth for the world's most valuable technology company.

Jobs, who has been on medical leave for months and last took the stage in March to present the iPad 2, will unveil the iCloud, a Web-based service that lets consumers stream music they bought to any Apple device, pitting it against rivals Google Inc and Amazon.com Inc.

That expansion into cloud computing is seen as crucial if the company is to stay competitive with increasingly popular open-sourced software, such as Google's Android operating system, according to analysts and investors.

The iCloud has the potential to make Apple's iTunes even more powerful, making it tougher for rivals to keep up, Sterne Agee analyst Shaw Wu said.

It looks like Apple will likely offer some base service for free, Wu said. Competitors, including RIM, Google, Amazon and Microsoft already have a hard time competing with iTunes as it is, but we believe will likely find it even tougher with iCloud enhancements.

The Moscone center in San Francisco -- ground zero in the launch of several iconic Apple gadgets such as the iPad -- was a hive of activity this week as workers put the finishing touches on banners featuring a giant Apple logo.

Apple has been busy wrapping up negotiations with major record labels to secure licenses for its iCloud service, which is also expected to include a revamped version of its little-known MobileMe storage service.

The licenses will help Apple introduce scan-and-match technology that scans a user's hard drive and provides access to music found there from the company's own servers.

Currently, Google and Amazon require users to upload their library of songs.

Some analysts say the iCloud has the potential to be a new model for media consumption, which could also spark more demand for Apple devices.

It could also enable the consumer giant to design new devices around the service, said Trip Chowdhry, managing director of Global Equities Research.

iCloud by itself will not be a billion dollar revenue opportunity, Chowdhry said. It is an enabling technology ... once you have things in the cloud, you can create new devices that (have not) been created right now.

The company, legendary for keeping its agenda under wraps, has been unusually open about what it plans to show at its annual developers' conference, a five-day extravaganza for developers that begins on Monday.

Apart from iCloud, Jobs will introduce software upgrades at the conference, including Lion, its Mac OS X computer operating system and the next version of its mobile operating system.

Still, fans, developers and Wall Street cannot help but feel the consummate showman may have a surprise or two up his sleeve because he typically takes the stage only for major events.

There certainly could be a surprise, said Current Analysis analyst Avi Greengart, who often attends Apple events. Apple is known for just one more thing.

Apple has used the WWDC venue in the past to unveil a new iPhone, but some sources told Reuters they did not expect the new model to appear until September.

But any appearance by Jobs, who survived a rare form of pancreatic cancer, is usually closely scrutinized by Wall Street, given that the fortunes of the $320 billion company are so closely linked with the co-founder.

One of the things that's going to be interesting to see is how much actual introduction Steve does and how much he is playing emcee, Greengart said. He may just come out on stage three times -- once to introduce the things, once to punctuate and once to close. Or he may be on stage for two-and-a-half hours, he added.

(Editing by Andre Grenon.)

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