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If you read articles about the WWDC Keynote (or just watch it) you'll see that iCloud will work thru the same Apple ID between iOS devices and computers. Pictures were specifically mentioned by Steve Jobs (thru iPhoto on Macs and Pictures on PCs), but I'm sure the other aspects of iCloud will work as well...
Again, at this point it's just a beta.
 
If you buy a track on your phone it'll download to iTunes, but I think you can actually do that now without iCloud.

Contacts, bookmarks, and Mail are handled the same way as MobileMe. You sign into iCloud in System Prefs and check boxes for Mail & Notes, Contacts, Calenders, Bookmarks, Photostream, Back to my Mac, Find my Mac. You do the same on your phone (with some diferences) and you're good to go.
 
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