Ballmer was more open Thursday.

"Novell pays us some money for the right to tell customers that anybody who uses SUSE Linux is appropriately covered," Ballmer said. This "is important to us, because [otherwise] we believe every Linux customer basically has an undisclosed balance-sheet liability."

"My reaction is that so far, what he [Ballmer] said is just more FUD [fear, uncertainty and doubt]," said Pamela Jones, editor of the Groklaw.net blog, which tracks legal issues in the open-source community. "Let him sue if he thinks he has a valid claim, and we'll see how well his customers like it."

Officials at Red Hat Inc., the leading Linux distributor, also dismissed Ballmer's comments. "We do not believe there is a need for or basis for the type of relationship defined in the Microsoft/Novell announcement," said Mark Webbink, deputy general counsel.

Red Hat has called Microsoft's legal threat a looming "innovation tax." It also said that it can protect its customers against patent claims.