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Case Study Number One:

It's a good video (but with slightly tacky music!) posted on Continental Airlines Windows Vista rollout using System Center Configuration Manager (SCCM) and their subsequent power savings - estimated at $2m by the time they are finished, across 18,000 PCs annually.  A good chunk of dollarinos there.

Check out http://www.microsoft.com/casestudies/casestudy.aspx?casestudyid=4000002271

Case Study Number Two:

The next one is Medtronic who also deployed Windows Vista across 43,000 desktops: they estimate a saving of around $750k annually.  Checkout http://www.microsoft.com/casestudies/casestudy.aspx?casestudyid=4000003027

So being a savvy reader you would have noticed there is a distinct cost saving different between case study number 1 & case study number 2 - why wouldn't 43,000 desktops achieve greater cost savings?

Although it doesn't say in the case studies, I believe the difference could be due the fact that Continental are using SCCM to shutdown & restart computers when they aren't used - Medtronic are only measuring the difference between Windows Xp -> Windows Vista: because in Windows Vista the Balanced power plan is configured as the default, it automatically has power savings over Windows XP.