Linus Torvalds just can't help but be a thorn in Microsoft's side.

First, he created an open source project that completely upset Microsoft's business model. And now, he has helped shoot down an important Microsoft patent in Redmond's crusade to wring licensing dollars out of Google Android and other versions of Linux.

Microsoft has coerced many Android phone makers into paying licensing fees for various Microsoft patents related to operating system design, and in some cases, it has actually taken legal action against such companies, including smartphone manufacturer Motorola. In October of 2010, it sued Motorola in federal court, and it filed a complaint with the United States International Trade Commission, or ITC.

Last December, Microsoft scored a victory when the ITC Administrative Law Judge Theodore R. Essex found that Motorola had violated four Microsoft patents. But the ruling could also eliminate an important Microsoft software patent that has been invoked in lawsuits against Barnes & Noble and car navigation device-maker Tom Tom.

According to Linus Torvalds, he was deposed in the case this past fall, and apparently his testimony about a 20-year-old technical discussion – along with a discussion group posting made by an Amiga fan, known only as Natuerlich! – helped convince the Administrative Law Judge that the patent was invalid.

Essex's ruling is merely an initial determination. It is being reviewed and could yet be reversed by the Commission. But if it's upheld, it could work against Microsoft as it continues to fight Android and other Linux-based systems that it believes violate its intellectual property.

Microsoft says that Motorola violated one of its patents, known as the 352 patent. This patent covers a technique for storing filenames with lots of characters in old filesystems such as the Windows FAT (File Allocation Table) filesystem that are designed to use very short filenames. Mobile phone makers use this type of technology so that their devices interoperate with other operating systems, including Windows.

"Motorola had found this posting of mine about long filenames used in a compatible manner with short file names... and it predated the Microsoft patent by three years," says Torvalds.

Torvalds says he did a video deposition last fall with Microsoft's lawyers, who tried to cast doubt on the dates of the newsgroup postings. "It was actually pretty annoying, just because the way they tried to cast doubt on what the date was. The lawyer went on for about five minutes: 'Are you really sure about this date thing?'"

At issue was a stamped newsgroup posting that came two hours after an earlier message on the same subject. Torvalds checked the posting against a second archive to make sure the timestamps were right, but, he says, Microsoft's lawyers kept pressing him. "At some point, I basically said: 'OK stop this stupid argument, can we go on to something else?'" Motorola and Microsoft's lawyers didn't respond to questions about Torvalds' account of the matter. A Microsoft spokeswoman couldn't comment on the case.

The Commission is just winding up the public comment period on its initial determination and a decision is due soon.

However, if the initial ITC finding is upheld, that would be "very serious," according to Eric Schweibenz, a partner with the law firm Oblon, Spivak who blogs about the ITC. The ITC is a trade body, not a court, so federal judges can ignore its findings. However, many of them pay attention to what goes on at the ITC.

An ITC ruling "would not be binding per se, but it would be highly persuasive," Schweibenz says.

That makes what happens at the Commission worth watching, says Keith Bergelt, chief executive officer with the Open Invention Network, a company that provides royalty free patents to Linux companies. "This is a patent that has a long history of concern and this is not just in the open source community, but from anyone," he says. "Because it seems so obvious."

Having the patent ruled invalid could cost Microsoft money, Bergelt says. "If you absolutely know that it's an invalid patent then you're not going to sign up for a license," he says. Existing licensees would also be likely to push for reduced payments.

But according to Florian Mueller another keen observer of software patents and the ITC dispute, Microsoft still has a lot of patents it can use when asking for licensing fees. "The fact that they use these [filesystem] patents a lot when they approach other companies doesn't mean that they don't have other patents to license," he says.

The ITC is expected to issue its final determination on the matter by April 20, but Mueller believes that the case will probably be appealed to the United States Court of Appeals for the Federal Circuit. Says, Muller: "Even April 20 won't put the ITC case to rest."