Verisign is migrating 2000 servers from a proprietary Unix to Red Hat Linux.:

    ...[the Verisign spokesman] also laid to rest the 
    criticism that Linux is not ready for prime time and 
    mission-critical applications, particularly at the 
    back end. "VeriSign is migrating several significant 
    databases to Linux, including Siebel Sales Force 
    Automation, Clarify sales support, and business 
    intelligence and decision support applications from 
    Informatica and Primus."

My friends in the business world stress that Linux is currently more a "threat" to proprietary Unix, than to Microsoft Windows. The savings is mostly in hardware - it's cheaper to buy commodity Intel boxes then Sun servers.

IBM has co-opted this threat by moving to make Linux it's standard OS on all its hardware. In other words, IBM is trying to make the operating system a commodity, while retaining its edge in quality hardware as well as software services. Over time, the succes of this strategy poses a huge threat to Microsoft.