Jon Udell writes:

IBM's mainframe Linux hosting service. A few months back I researched and wrote a story on mainframe Linux. ... I continue to find this technology alliance fascinating. Moving parts are the enemy, in my mind. The fewer the better. Provisioning a server farm in software, rather than as a collection of physical blades, seems like a great idea. The mainframe always had the raw virtualization capability, now in Linux it has something that's really worth virtualizing. One outcome, as this story notes, is a new kind of competition for the RackSpaces of the world: ... [Jon's Radio]

We have 4 large mainframes in our primary data center and the disaster recovery site.  Jim Calloway and his team have been playing around with Linux on them for about a year now.  So far, I've yet to see an ROI that makes sense, but then again, I've yet to see an ROI that takes into account total cost of ownership (TCO) including the reliability and availibility costs.  We run a tiered support model to provide reliability and the impact on the tiered support system of using the mainframe to run Linux would be interesting to analyze.  

As an aside to Jon's comments, we do run Oracle on the mainframes. In my previous life, we ran Veritas on paired Sun 4500's to provide the required databse reliability and that was just a flat out pain in the neck.  There's a big question in my mind as to whether Veritas was any help at all.  Often it made simple failures into bigger ones.  Running Oracle on a mainframe is the only way to go, if you can afford the mainframe or happen to have two or four laying around.