The synchronization as described by Apple is very prolific, and will make Mac users who own iPhones and iPads (and that's a lot of you) able to access all of this content no matter which device you may be using. Such access is a nice thing, but it requires the constant movement of data around the user's personal cloud. This has the potential to eat up the mobile data caps in place on iPhones and 3G iPads.
Every time a document is created on one of these devices, it is shot to all the other devices owned in the background. One document is pushed to two devices for those who use a Mac, iPhone and an iPad. It isn't just moved around once, with the new Revisions in OS X Apple is keeping full versions of each documents, and it sounds like those will be pushed around to the other devices too.
That is just document creation; when you buy a song (or album) on iTunes on one of these devices, it's getting pushed to the other two. Add in backups and document synchronization and the possibility is very real to have data flying around a lot of the time. This will all be subject to these data caps imposed by the carriers, so iCloud users may start finding themselves in overage territory.
The iCloud service sounds wonderful, and I am pretty sure I will be giving it a try when it's available in both OS X and iOS 5. I do think it's worth keeping an eye on mobile data usage once it gets rolling.
Image credits: Apple; Flickr user swimparallel
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