When I first posted this I wasn't aware that iCloud.com was an under-NDA developer preview. So I've removed the screenshot and some other stuff. If you're not a developer, try this.
I have been poking around in Apple's beautiful new iCloud.com page a bit. Judging by the source code that is freely accessible to anyone, it recreates SpringBoard in SproutCore, making the icons behave and move almost the same way they do on iOS. It uses SproutCore version 1.6.0.rc.1, statecharts and modules.
The interesting thing is that the application is called "CloudOS". That might possibly hint to more, or it might just be an ambitious name for the project. You can find the source code of CloudOS here. You might want to put it through something like JSBeautifier to make it readable.
Note, however, that you'll get an error when you click on the github icon. Enjoy poking around, and if you find out how to fix it, fork my gist or leave a comment!