Despite what IBM claims, COTS support for the mainframe is actually pretty limited (and often far more expensive), even if the MF is running zLinux. The mainframe is also a huge single point of failure and failover solutions to a second data centre are again, really frickin expensive. I'm not a mainframe expert, but I'm not convinced by this story.
Wasn't there a news the other day that NASA has completely eliminated their mainframe system?