Microsoft states, that Linux violates 235 of their patents. I assume, that this number is greatly exaggerated. But: Even when your enemy has 235 bullets, one is sufficient to kill you. And the interesting question is: Can you dodge 235 bullets? Interesting times ahead.
There even seems to be a raging cold war regarding this topic. Roger Parlof of Fortune wirtes in Microsoft takes on the free world:
So if Microsoft ever sued Linux distributor Red Hat for patent infringement, for instance, OIN might sue Microsoft in retaliation, trying to enjoin distribution of Windows. It's a cold war, and what keeps the peace is the threat of mutually assured destruction: patent Armageddon - an unending series of suits and countersuits that would hobble the industry and its customers.