Make it a threesome. On Thursday, WPP Group (NASDAQ:WPPGY) joinedInterpublic (NASDAQ:IPG) and Omnicom Group (NYSE:OMC) in buying advertising space from TiVo (NASDAQ:TIVO) on behalf of its clients.

Financials details weren't disclosed, but it's a good bet that TiVo stands to make millions from the arrangement. Advertising, after all, is a multibillion-dollar industry, and some of the world's largest firms have cast their lot with TiVo, including Sony (NYSE:SNE), General Motors (NYSE:GM), and Kraft Foods (NYSE:KFT).

But this deal is less about revenue and more about obtaining a competitive advantage. Seriously, do you think it will be long before French ad firm PublicisGroupe (NYSE:PUB) decides it, too, must be able to access TiVo for its clients?

If not, then what's the alternative? Answer: There isn't one. TiVo boasts a fat patent portfolio that includes protection for advertising on a DVR. With EchoStar (NASDAQ:DISH) already on the business end of a losing legal tussle, I doubt many others will rise to challenge the DVR pioneer in the courts.

Frankly, my biggest fear with TiVo has nothing to do with copycats or the courts. Nope, my biggest fear is Nielsen, which in November plans to roll out new boxes for rating ads. They'll also be able to track ad viewership for prerecorded shows, which, in turn, will provide the first real data on the so-called "TiVo effect," which has been blamed for a prolonged malaise in TV advertising.

If Nielsen's data reduces the "TiVo effect" to myth, look out -- the business and the stock could fall apart quickly. But I don't expect that to occur. Advertisers, after all, have ways of finding out what works, with or without the aid of technology. If three of the biggest ad firms are willing to spend millions to hedge their bets, as they have with TiVo, you'd better believe TiVo has developed something worth worrying about. Better stay tuned.

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Fool contributor Tim Beyers still thinks TiVo is the best thing ever to happen to TV. Tim owns shares of Interpublic. Find a complete list of his holdings by checking Tim's Fool profile . Kraft is a Motley Fool Income Investor pick. The Motley Fool has an ironclad disclosure policy .