We still don't know what it's for, and it hasn't even officially opened yet, but multiple sources are claiming that Apple is already planning on doubling the size of its North Carolina data center. A local realtor informed Data Center Knowledge that "site work is being performed," which seems to confirm rumors from the Charlotte Business Journal and All Things Digital from last week.
Much speculation has surrounded Apple's intentions for the site since construction began in 2009. The most popular theory so far has been that Apple is planning on launching a streaming music service for iTunes, but with some music industry figures reluctant to even allow Apple to extend the length of its song previews, a subscription-based iTunes may still be far down the road. The data center should be opening soon (at least its first stage, by the sounds of things), so in theory, we should know what it's for fairly soon, too.