A cloud. Photo Karen Ka Ying Wong / Flickr

Apple has confirmed maybe the worst kept secret in Silicon Valley: it’s been working on a cloud service, and will announce it at the June 6 Worldwide Developers Conference.

So now we know annual developer’s conference will unveil “iCloud®, Apple’s upcoming cloud services offering,” but we don’t know yet what it is, or what exactly will reside at the domain name it may have bought for $4.5 million in April.

iCloud could be the mythical iTunes streaming service, a possibility that is looking increasingly likely now that Apple has most of the major record labels signed up for it, in a space pioneered by Amazon and Google, which has no such deals. Or it could be Apple’s version of DropBox, the amazing sync and storage service that currently glues together the whole iOS ecosystem along with its desktop service.

Or it could be yet another abortion of an Internet service, like Mobile Me, iDisk and Ping before it.

Whatever it is, it’ll have to be good. DropBox is already the default file system for iOS, and can be used by any developer, on almost any other OS. If iCloud is to succeed, it needs to be just as available to developers. ITunes streaming, too, needs to be something special. A “cloud locker” service like Amazon and Google’s is almost pointless, and we already have amazing streaming services like Spotify and Rdio.

Still, this won’t stop endless speculation during the next week, as pundits work up theories like a shaving brush works up a lather from a tiny, almost non-existent nubbin of soap. And then on Monday, boom, we’ll know for sure.

Apple also said the keynote will be delivered by Steve Jobs, who is currently on medical leave, and that the WWDC will also focus on the new version of OS X 10.7 Lion, and the upcoming iOS 5.

Apple to Unveil Next Generation Software at Keynote Address on Monday, June 6 [Apple PR]

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