Hard to say...
...if this really fits IBM's business strategy. They do have a lot invested in Windows on their X series servers so I'm not sure they have anything to gain by running Windows on the mainframe unless Microsoft is willing to pony up a lot of partnership. Right now the IBM mainframe line is pretty much a layered SOA setup with Linux and evidently possibly Solaris running on peripheral servers which can be used to run WebSphere and other web apps while the central mainframe processors running z/OS are mainly focused on DB2 and similar backend high availability vertically scaled applications. Thus it's not clear to me that IBM has much to gain by running Windows on their mainframes. The can run it on Nehalem instead at much lower cost and WebSphere is horizontally scaled so having multiple four socket blades is not an issue there. It appears to me that IBM will probably keep Windows on X class blades though if there is evidence they're creating a new "reduced capability" processor like the ones they use for Linux on the mainframe then this would have more credence.