You're WONDERING what they're doing?
Apple has a MASSIVE data load to deal with. They're continuing to not only add content to iTunes, but they're also improving the resolution of that content, and the bandwidth requirements for delivering each file as well. Combine that with Apple's continuing movement into personal websites, information and file sharing, data backup, hosted services, and more, yea, they need a MASSIVE datacenter. Also the interconnections between iTunes and Twitter, facebook, etc likely has some significant server side presence, and expanding their server side role in the iPhone push technology is happening fast.
Also, Apple probably could use a place to work on xGrid systems and massively parallel cluster applications and other cloud based services and application virtualization. Further, Apple is rumored to be working on both their own search engine as well as their own financial transaction system. A hulu-like product is also completely possible in the very near future.
Just the physical storage pool for 1080p content covering every major movie and TV episode released in a calendar year is amazing, not to mention taking their more than a million song database and converting it all to lossless rips.
I can't even imagine the aggregate switch capacity Apple needs, let alone the bandwidth costs. I would not doubt that of that billion, nearly a 100 million in in ISP connectivity over 3-5 years, and another 100million in switch hardware, before we talk structural and traditional server costs...
