A reader writes "The new Z-series mainframe for Linux, which costs $400,000 and is aimed at processing transactions at large businesses, is IBM's first mainframe computer sold without IBM's traditional z/OS mainframe operating system. More info at the IBM zSeries page"
This is something that IBM and others of Big Iron vendors of *NIX have said - as Linux grows in maturity, they want to replace their *NIX with Linux. However, there's still work to be done
in that area.