When techie websites reported on Microsoft’s Jan. 19 suit accusing TiVo of infringing two Microsoft patents in its digital video recording systems, they were astute enough to tie Microsoft’s new filing to TiVo’s ongoing DVR patent litigation with AT&T in the Eastern District of Texas. “Microsoft has AT&T’s back” was the headline on Engadget’s widely picked-up story about the suit. “The DVR patent fight just got a little crazier, with Microsoft greasing itself up and jumping into the arena–seemingly to preserve the honor of AT&T,” the story says.

Well, it’s a little more complicated than that. Microsoft’s stake in the litigation isn’t just for AT&T’s honor. It’s also, quite possibly, for the potential damages AT&T faces in the TiVo case.

Here’s why. Last spring, TiVo sued both AT&T and Verizon in federal district court in Marshall, Texas, accusing them in separate suits of infringing three TiVo patents in their television playback services. AT&T’s service, U-Verse, relies on Microsoft’s Mediaroom software. On January 15, Microsoft moved to intervene in TiVo’s suit against AT&T. Microsoft’s complaint in intervention, which seeks a declaratory judgment of non-infringement, simply argues that the software giant has an interest in the litigation because TiVo’s allegations involve a Microsoft product.

But Microsoft’s brief in support of its motion has a very interesting additional piece of information: AT&T’s lawyers at Baker Botts, the motion says, have demanded indemnification from Microsoft in the TiVo case. Microsoft senior attorney Stacy Quan submitted an affidavit attesting that AT&T wants Microsoft to back it against “the allegations and claims of patent infringement by TiVo Inc. directed to U-verse products and/or services.”

TiVo general counsel Matthew Zinn told the Litigation Daily that it seems clear the new Microsoft suit against TiVo “appears to be related to the AT&T situation….They’re defending their customer.” Noting that Microsoft told Bloomberg it’s “open to resolving this situation through an intellectual property licensing agreement,” Zinn said, “It’s fair to assume [the new suit] is part of the same strategy.”

Microsoft is represented in both the AT&T intervention and the new suit against TiVo by Chad Campbell and Lauren Sliger of Perkins Coie. Campbell didn’t return our call for comment.

TiVo is represented in the AT&T case by Irell & Manella; Irell’s Morgan Chu told us that he will defend TiVo in the new Microsoft suit as well. TiVo is using Robins, Kaplan, Miller & Ciresi in the Verizon suit.