Going back to the data capacity density topic at the beginning of this thread...
I manage a certain Dell EqualLogic iSCSI device. After accounting for RAID 50 redundancy and two hot spares, it'll serve up 23.23 Terabytes in 4U of rack space. (All capacity numbers in this post use the base-10 nomenclature, so 1 Tb = 10^12 bytes.) Given that 10 of these will fit in a standard 42U rack, that's 232.32 Tb per rack. To store 1 Exabyte, you'll need 4304 racks. At four square feet per rack, you're looking at 17,217 square feet of floorspace. (Another way of thinking about it is you'd need 86 rows of 50 racks. With a row every third floor tile, you're looking at (50*2) * (86*3*2) = 51,600 sq ft.) That's large but nor crazy large. Obviously that doesn't include power, cooling, and other infrastructure needs like redundancy, network, servers, tape libraries, office space, etc., but Amazon's Oregon datacenter is 116,000 sq ft I've read, so storing 1 Exabyte there isn't a floorspace issue.
Also, Apple claims "over 20 million songs" available via iTunes. At an average of 4 minutes per song (an assumption based on a Google search) and 256 Kbps, that's only 1.2288 Petabytes, or a little more than four racks worth of storage using the above device.
And finally, if we assume 200 million iOS devices each provisioned with 5 Gb of iCloud storage, we're talking the 1 Exabyte number. Not including redundancy which increases requirements, deduplication which decreases them, etc.
My guess for Apple's usable data storage capacity at their datacenter? One or two hundred Petabytes. YMMV. <shrug> (Feel free, btw, to check and correct my math and assumptions - I want to be factually accurate as possible and would hate to be off by 1000. Except for my final guess of course.

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