I'm sure it is possible, but if it is true, why hasn't Microsoft sued anyone or revealed which parts of Linux or the software that runs on it infringe their IP rights?
The response to Ballmer's statement from the open source/free software communities has been overwhelmingly defiant. I think they want Ballmer to back up his words and are not at all afraid of a lawsuit. I cannot make up my mind whether this is just bravado or based on facts. I sense the former but am assured by my FOSS developer friends that if in fact the threats are based in reality, the various developer communities will strip out the infringing code and re-write it in very short order.
One way or another, this is going to get interesting. Remember; Red Hat is suing SCO simply because SCO impugned Linux. RH is seeking a declaration in court that Linux does not infringe any of SCO's intellectual property. It would not surprise me if either they or another Linux company (Canonical comes to mind, here) sues MS seeking the same outcome. There are many Linux companies with deep enough pockets to sue Microsoft and one or more of them just might.
And more than one pundit has wondered aloud how suing over Linux would affect Microsoft's bottom line. Alienating a significant portion of your customer base is never wise and it is a certainty that a very significant portion of Microsoft's enterprise and government customers are running Linux in one form or another.