How quickly we forget ... its only just over a decade ago now that Microsoft was caught out stealing GPL licensed code for use in Windows - specifically the TCP/IP networking components. The EFF backed the class action against them, and Microsoft ended up losing.
Then, just five years ago SCO tried the same sort of thing against IBM, claiming that Linux was in many ways directly copied from SCO-Unix and that IBM staff were responsible for copying it. It took a few years, but when you decide to sue a company that has no fewer than 20,000 lawyers on their payroll, there is really only one way it can go.
This sort of sabre-rattling happens all the time. Usually it is just that - merely a pose that has been struck for publicity's sake. On the few occassions that software patent issues have gone to court, it has almost always ended up as an expensive disaster for all parties concerned.
In this case I think it is a serious miscalculation by MS; they felt that by claiming that Linux copies from Windows they could win back waning support by playing the victim, and make hundreds of middle-management types who have the power to choose which OS their companies use just that tiny bit more nervous about the jump to open-source. And I'm pretty sure it will fizzle out long before the lawyers even get their wigs on.