In ZDNet GreenTech Pastures blog, Microsoft's latest data center in Northlake, IL is discussed as a Green Location.

From a tech industry standpoint, one big side effect of the software as a service (SaaS) movement will be the build-out of massive data centers capable of scaling to handle ever-growing user traffic. That doesn’t sound so green now, does it?
No wonder Google refuses to reveal just how much power its operations consume on a worldwide basis. In all fairness, arch-rival Microsoft also holds certain information related to its data centers pretty close to the vest. But the big Seattle-based software developer was eager to trumpet its move last month to invest about $500 million in a major data center located in Northlake, Ill.
The facility, being developed by green data center design expert Ascent in partnership with real estate company The Koman Group, will be Microsoft’s single largest data center when it comes online in the June 2008 timeframe.

Data Centers are interesting distribution logistics problems applied to information instead of goods. Chicago given its history of being a hub of commerce works well for information distribution.

Phil Horstmann, CEO of Ascent, says his company looks at each data center as an intersection of a lot of vitally important utilities—namely power, water and connectivity. The Northlake facility will feature a number of design innovations, such as an arrangement to tie right into nearby electricity transmission lines. Because Chicago is a deregulated market, his company can buy from 15 different utility companies.

Ascent's marketing site for the Northlake Data Center facility gives you an idea of the additional factors which made the site appealing.

Other green features of the area are cold air allows airside ecomomization for cooling, and Chicago power has a low carbon footprint. This Computerworld article discusses the most energy-efficient locations.