Once you have that VMware cluster you can run your choice of 70+ operating systems and millions of apps on it. Can you run Exchange on a mainframe? Sieble? Your existing billing and accounting app?
Well, you can run whatever runs on Linux on top of a mainframe. And if you're a Fortune 500 corporation, chances are your existing billing and accounting applications are *already* running on a mainframe. That is, after all, what the old girl is built for.