A lawsuit does not sound like a good idea. Microsoft has a top notch legal team, the wrongdoing was committed by an outside vendor, and there are three countries involved, each with their own IP laws. A suit will be long and expensive, and MS will do everything in their power to put all liability on that outside vendor, who probably has meager assets to pay damages. Even if they win, the reward will be attempting to collect damages from an uncooperative company somewhere in China.

Plurk: you got your goodwill and free press, don't push your luck. All you will get from a lawsuit is a five year headache and huge bills from your lawyer.