Don't act so dense. Their intention was never to sue. If they did, Red Hat would have ended up in the soup years ago when Microsoft trolled this out the first time.
No, what Microsoft wanted was for a few distributors to blink. They got that. Novell, Linspire, and Xandros blinked, though questionably their motivation for blinking seems suspect anyway. Novell has de Icaza, so is it really a mystery they jumped at the chance to sign this deal when the biggest public Micropologist (Who called OOXML "superb" and embraced .NET as the "next great thing from Microsoft?") works for them? Linspire, from its beginning, has always wanted to be like Microsoft. Xandros is the only one I don't understand. They probably just fell for it.
Microsoft wants distributors under their thumb to inject poison into Linux and FOSS. That's what Mono and Moonlight are, in my opinion. And the only guy who actually wanted to develop a .NET/Silverlight implementation for Linux was non other than everyone's favorite Micropologist, yes, AGAIN, de Icaza.
This is just one example of Microsoft attacking Linux. TomTom is not a reliable enough example. First off, I don't think it had anything to do with the fact that TomTom uses Linux but that TomTom didn't licence vfat. They've done this to non-Linux distributing companies that violated the exact same patent.
Maybe spreading FUD campaigns against Linux like Get The Facts or this Best Buy thing in much more recent history.