Jack Messman in an interview with Computerworld may imply that it is not yet:
http://www.computerworld.com/softwaretopics/os/story/0,10801,80359,00.html
Many Linux users would disagree. Even if Windows is marginally more efficient or secure, many companies just might end up using Linux to save money and to have greater control and compatibility->IT world
This is explored in point #6.