I can speak from experience here. My company has 13 offices and about 180 employees. We run a SuSE Linux mail server and RedHat servers for samba. Still using Win2k and XP for the users, as we have to use Windows specific software like AutoCAD. We have two IT guys for the whole company. Our servers are all comodity PCs. The IT guys migrated to Linux after M$ pulled the NT lisences from the company, which is what the servers ran before.
Since we buy our laptops from regular dealers, they come with Windows preinstalled anyway, so we do not have any "deals" with M$.
The company is looking into running Ximian + Redhat + OpenOffice for users who do not need AutoCAD, but even I have to run stuff life AquaSolve, which only runs on Windoze and many states require the use of specific custom software for work in Risk Based Assesments, and that software only runs on Windows. Iowa's software is writen in Visual Basic
But moving the backend of small companies to Linux is cake, and M$ can be damned about supporting 98, NT, ME, whatever- if it came with the computer, you can still run it, with or without their blessing.