Taking the snapshot look, Apple notes that mail, contacts, calendar, bookmarks, Find My iPhone, and Back to My Mac will all be available in iCloud. Users of those services in MobileMe will be able to transition them over and continue using them under the new iCloud banner. Apple hasn't explained if they'll be feature-for-feature copies, or if select features of each individual service will be lost/gained with the switch.
Some services didn't make the iCloud cut, though, including iWeb Publishing, Gallery, and iDisk. iWeb Publishing, for example, will run through June 30, 2012. After which, users will need to seek out new Web hosting platforms. Users' MobileMe photo galleries also will be available through June 30, 2012, but Apple cautions users of this service to back everything up to a local computer. The same applies to iDisk.
The aforementioned services will be replaced in one form or another by new services, such as iTunes in the Cloud, Photo Stream, and Documents in the Cloud. These will allow users to manage and access their music, images, and documents from whatever compatible device they may have on hand.
Existing MobileMe users (at least those who had an active account on or before June 6) will have access to their @me.com or @mac.com email addresses, contacts, calendar, and bookmarks, all of which will be available through a Web-based portal. The Web tools means users can manage their settings and preferences from the desktop.
The bad news is that iCloud ditches a lot of the syncing features of MobileMe. Things that can't be synced include: Mac Dashboard widgets, keychains, Dock items, and System Preferences. These items will be available to MobileMe users until June 30, 2012, or until they transition over to iCloud. Once a MobileMe user makes the move to iCloud, however, all these syncing features will be lost. It is worth pointing out that this set of tools was made to sync user preferences and settings across Apple computers, and not necessarily mobile devices. (That's not to say they didn't come in handy.)
iCloud will support automatic downloads for app and book purchases, and backup and restore, which means that iPhone and iPad settings and data will be safe. Users can set their devices to sync/back-up automatically via Wi-Fi and when plugged in. Most iCloud users shouldn't have to worry about their device preferences and settings overmuch, because they'll be recoverable.
iCloud will be free to all users of iOS 5 devices and the upcoming OS X 10.7 Lion operating system.
If you have more specific questions about what will happen to your MobileMe data, be sure to check out Apple's FAQs and in-depth articles on the subject.
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