While on the surface this may seem like a stretch, the reality is that the technical solution is solid, the economics are compelling, and there is a quiet group of companies actively pursuing Windows/Intel migration options for both mainframe and midrange systems. This trend has become more urgent due to recent end-of-life announcements from IBM on some of their older systems that still have a significant installed base, and the ever-escalating cost of maintenance and change management for mission-critical apps running on "big iron."
There is a specialized group of ISVs and system integrators that specialize in certain aspects of mainframe and midrange migration. In addition, Microsoft has recently released a new edition of Host Integration Server, allowing .NET systems to interoperate with mainframes, and mainframe apps to be extended via web services and other newer technologies.
There is a variety of great information on the following sites - check them out for technical info, partner listings, and case studies from customer who have made the switch.
Infoworld Article: Is it Time to Retire Big Iron?