A new IDC white paper is available at http://download.microsoft.com/download/5/B/D/5BD5C253-4259-428B-A3E4-1F9C3D803074/IDC_Business_Critical_Workloads.pdf which documents what’s happening in the world of x86-based Servers; why they are THE platform businesses choose for their business-critical workloads.
This is an objective look at how businesses are taking advantage of new advances in server hardware. The paper also goes into detail about the kinds of workloads being tackled with this kind of hardware married with Windows Server as the OS platform, and the benefits businesses are getting out of it.
In this report they explain "IDC finds that BP workloads are progressively shifting from mainframes and host servers based upon RISC and EPIC (Itanium-based) architectures to x86 servers," (on p. 6).
There is also a webcast of Al Gillen where he describes the research.
The report expects to see x86-based machines increasingly running business processing (BP) workloads and credits improvements in x86-based server hardware technologies -- plus scalability enhancements in Windows Server 2008 R2 and SQL Server 2008 R2 -- as leading to a technology shift that Windows Server Becoming a Mainframe Contender.
Another report is available at http://redmondmag.com/articles/2010/04/14/windows-server-becoming-a-mainframe-contender.aspx