Linux is a threat to M$ but not on the Desktop[.
The Back End Servers in the bigger places and all of the ISP's run some form of Linux mostly Red Hat or SUSE if what I have seen is anything to go by. These are the Servers that M$ wants to get Windows onto and are exactly what Windows isn't suited to.
Those Servers are a big slice of the market in that area and are predominately some form of Nix.

Col