InformationWeek has published a report  on a study conducted to guage the use of open source use in businesses. Those that responded cited Microsoft security issues as main reason they use open source.
Quote, "Use of the Linux operating system will continue to grow as businesses seek a lower-cost option to Unix and a more open and flexible alternative to Microsoft's Windows. However, the future of open-source products in the enterprise is hardly a given. Compatibility with existing software is the hurdle to overcome with Linux on PCs. Other common complaints when the operating system is used on servers, laptops, and desktops are problems related to multiple Linux versions, poor documentation, and hardware-support issues."