MS (like everyone else I suspect) do eventually force updates on people by discontinuing support (unless an extended support contract is paid for, and those are expensive enough to strongly encourage companies to upgrade).
(EDIT: I may have confused the names of "extended" support and "custom" support. See Larry Osterman's post two below and the page he links to for a more accurate description.)
I think it was this which finally convinced the last big company I worked for to get off their behinds and upgrade from NT4 to XP.
(That was a couple of years before Vista came out. They didn't avoid Vista on purpose.)
(This is a place which still uses the crime against computer science known as Lotus <spit> Notes for their email so you can imagine they're reluctant to upgrade their non-business software no matter how much pain it causes people every day.)