Microsoft's talks the language of interoperability but its hot and cold attitude to open source after claiming a year ago that Linux violated hundreds of its patents doesn't look to have worked.

It was just over a year ago that Microsoft dropped a bombshell of a claim: users of Linux and open-source software were unwittingly violating as many as 235 Microsoft software patents.

"This is not a case of some accidental, unknowing infringement," Horacio Gutierrez, Microsoft's vice-president of intellectual property and licensing, told Fortune magazine at the time. "There is an overwhelming number of patents being infringed."

Since then, critics say Microsoft has played the “good cop, bad cop” routine with the open-source camp.

For example, promising not to sue customers of 8 vendors that had signed cross-licensing deals with Microsoft for potential open-source-related violations? Good cop. But continuing to refuse to publicly reveal what those alleged patent violations are? Bad cop.

Here's another: Announcing in March that open-source developers will now be able to use hundreds of Microsoft software protocols without a license for non-commercial use? Good cop.

CEO Steve Ballmer telling customers of Red Hat, which has refused to take out a license, last October that they "have an obligation to compensate" Microsoft for IP violations? Bad, bad cop.

Gutierrez, interviewed late last week, says Microsoft's hot-and-cold engagement with the open-source community is neither "intentional" nor "inherently contradictory."

"We spend $7 billion (£3.5 billion) a year on research, development and cranking out innovations," he said. "We need to protect our innovations against people who infringe upon them."

But legal eagles in the open-source camp argue that Microsoft's moves, even its ostensibly positive ones, have done little to bolster its patent claim.

"Claiming you have IP that folks are infringing isn't the same thing as proving it," wrote Pamela Jones, author of the open-source legal blog "I think they [Microsoft] are in a weaker position *because* they did the [cross-licensing] deals. It makes them look needy, like they can't make it any more without Linux."

"The [legal] threat [to open-source] is no greater" today than a year ago, according to Mark Radcliffe, a lawyer with DLA Piper's Silicon Valley office and the general counsel of the Open Source Initiative, which oversees the approval of open-source software licenses, in an e-mail.

Take Redmond's attempts to persuade vendors to sign cross-licensing deals that include protection from potential open-source patent lawsuits by Microsoft.

Besides Novell and Fuji-Xerox, which both had signed deals before “PatentGate”, other cross-licensees include Asian consumer electronics makers Samsung Electronics, Kyocera-Mita and LG Electronics, and a trio of smaller Linux makers including Xandros and its Scalix subsidiary, TurboLinux and Linspire. The overall number taking our patent agreements though remains at 8.

In an interview last week, Gutierrez said that cross-licensing is vital because "customers don't want to buy an IP problem."

Moreover, it sets the necessary groundwork so that Microsoft and its partners can comfortably work together to make their respective products interoperate.

"Good fences make for good neighbours," he said. "This is not a religious issue, but a very practical one."

But critics claim cross-licensing is a sneaky attempt to get open-source vendors to de facto admit that their software does violate Microsoft patents. Not that they think it actually bolsters Microsoft's case.

"The signing of cross licenses does not mean that the patents are valid," Radcliffe wrote. For instance, licenses can be signed "for a variety of commercial reasons to reduce risk" or to give one Linux vendor a perceived edge in the market.

Jones is blunter.

"I am not aware of any relevant Microsoft patents that have been court-tested. That would be the only way to strengthen Microsoft's claim that there is anything actually infringing," she wrote. "All they have proved is that some will cave rather than find out in court."

The last Linux patent protection deal, with TurboLinux, was more than six months ago.

But Gutierrez promised more cross-licensing / patent protection agreements soon. There are "ongoing discussions with US software makers," he said. "They just can't be timed from a PR perspective."

Gutierrez thinks even vocal resisters such as Red Hat will eventually come around, once they "recognise that there are opportunities that they are missing out, as customers are increasingly demanding vendors come together and tackle interoperability challenges together," he said.

Red Hat did not reply to a request for comment. But Groklaw's Jones is sceptical.

"The world wants openness and interoperability, up to a point," she wrote. "I don't know anybody who wants to interoperate with Vista."

Moreover, "Red Hat is making money at least in part because it *didn't* sell out to Microsoft, in my opinion," she continued "Even if they [Microsoft] had a valid patent, going after Red Hat is like threatening to kill Dorothy's little dog Toto. You can't do it and have people like you."

DLA Piper's Radcliffe argues that the fact that most corporate networks have both Microsoft and open-source software, which Microsoft asserts leads to increased demand for interoperability, also immunises open-source vendors from Microsoft's legal threat.

"Most Linux users are not concerned about this threat," wrote Radcliffe. "Since they are also Microsoft clients, they believe it is unlikely that Microsoft will sue their own customers. We have not seen a significant shift from the 'licensed' Linux vendors to the 'unlicensed' ones."

Todd Weiss contributed to this story.