Yes, referred to as a "patent cross license agreement". I'm pretty sure Microsoft already had one with IBM and Sun, and now have one with Novell. If Red Hat doesn't have enough patents to enter into such an agreement, nor is willing to purchase the right to use Microsoft's patented technology, Microsoft could sue if they thought the reward was worth it, just as IBM is suing Amazon.com now for patent violation. I don't expect it though, at least not anytime soon, Microsoft doesn't have a history of "agressive" patent lawsuits, I expect smaller more agressive companies will eventually start suing Red Hat for patent infringement instead, like Firestar recently did over their Jboss middleware.
Which is what, exactly? Even Ballmer's pulling a McBride. You'd think he would've learned from SCO's mistakes.
MS or others can sue until the cows come home but it ain't going to change the reality on the ground. Do you really think it possible to sue linux out of existence? Now it would be like trying to claim that you owned the idea of binary logic or interrupt driven IO. Maybe you do, maybe you don't, but the world is going to go on using it regardless.