OK start with the Red Hat License agreement. Have any of you read it? In a nutshell, it says that anywhere you run Red Hat on a server it requires purchase of a subscription. And you can't buy a workstation subscription for a server, it has to be a server subscription. Subscriptions are based on 'sockets', which means CPU in real terms.
A 2 socket RHEL license costs $349/year on the 'self-support' model, and a 4 socket license costs $1,598 per year for standard subscription. Compare that to Windows Server 2008. The cost is $722.99 on CDW right now for W2K8R2 Standard. BUT, that's a one-time cost. And you get patches for free, regardless if you have a support contract or not. Figure that a Windows Server version may be supported for 10 years or more (2003 will run through 2015.)
Red Hat: $350 per year for 12 years = $4,200
Windows Server: $722 total, for 12 years = $722
That ends up costing you six times as much in license and support to run RHEL. Extrapolate that across hundreds of servers, and it becomes a monstrous expense. 500 servers = $174,500 per year. And yes, I assume you are going to re-buy a license for the new Windows Server one or two revs into the future.
THIS is exactly why we are not using RHEL in a highly compliance-oriented industry, and why we elected to go with CentOS. In the end we're going to be doing the support ourselves anyway, and Red Hat's cost structure is outrageous for what you get.