even if a migration to a Linux based platform takes longer than a migration to the next version of Windows (not a given, some Windows migrations have been known to take a very long time), once it's done the Microsoft licencing headaches stop. And the virus/worm headaches stop. And the stability headaches stop. And the systems can be customized so that other headaches stop too.
Rumour has it that it takes fewer Linux administrators than Windows administrators to manage an equivilent number of machines. And these Linux administrators have the time to do useful work. As opposed to patching/rebuilding/devirusing etc...
GrokLaw covered Ms. Didio's article. You can read about it as [link|http://www.groklaw.net/article.php?story=20040407003857488&query=yankee+group| Cranking Out the FUD -- and Some AntiFUD -- on TCO and Security]