OS X Server on IBM servers is a good idea if Apple charges enough for it.
Apple currently subsidizes OS development with hardware sales, but if IBM could put OS X on some big iron, there's room to give Apple the OEM licensing fees they require.
I do think IBM could provide a service that Apple can't. Big, robust, servers with a service plan to support big purchases.
The only fly in the ointment I can think of is that Apple may have to penetrate the enterprise market...