As you can see on the image on the left the spelling checker of MS Office doesn't know the word Linux. A little bit confused I searched for the word in the Duden and as expected the Duden knows Linux.
The image was taken with Word 2003 in Wine on an Ubuntu machine. I'm interested whether this occurs on Office 2007 or Office 2010 too. But I've got just a old MSO 2003 license with which I've taken the images. So if you've got one of this office suites it would be nice if you could test this and would leave a comment.
— Anyway LibreOffice's and OpenOffice.org's spelling checker of course knows the word Linux.