For example, a source familiar with the GroupM infrastructure said that planners within the group's U.S. division were warned to stay away from the tool. The reservations stemmed from the perception that companies using Google's Ad Planner could not be sure that the search giant wouldn't share their campaign planning data with competitors.

But according to Alan Schanzer, managing partner, MEC Interaction, North America, no such order has been given. "That's not true," Schanzer said. "The system is being evaluated, but that's as much as we're willing to elaborate on right now. It's too early to make any definitive statements."

A spokesperson from sister shop MindShare Interaction echoed Schanzer's response. "No one has been forbidden to use it," the spokesperson said. "We're evaluating it just like everyone else. After all, the tool has just been released."

Still, some insiders can see why Google's Ad Planner might be met with tempered interest. "I can understand the concern that an agency might have with Google having more access to their and their clients' info," said Noah Elkin, VP of corporate strategy at Steak Media. "At the same time, it provides access to more data, and it's free. I wouldn't recommend relying solely on it, particularly because it's so new and the store of data is limited relative to a comScore or Nielsen, but it could be one of the many tools to use in the toolset."

Ben Kunz, director of strategic planning at Mediassociates, agreed. "We're open to any tool that leads to more effective planning," Kunz said. "With the best planning, you triangulate and weigh your data sources against each other. On one hand, the panel approach is a little archaic, so Ad Planner has the edge. But from a quick scan of the tool, the consumer profiling isn't very sophisticated. With a panel, you can find out if your target would purchase x, y, or z, and with Google you can't do that."

Kunz said that with time, Ad Planner could become a trusted resource, much like Nielsen, comScore, or even Quantcast. "I don't think it will replace any of the other data sources right now," he said. "It's not there yet. But it may be the Holy Grail in a year, and if so, I wouldn't want to be on the measurement side of things watching this come out. It must have some people concerned."

Indeed, comScore's stock took a 23% dive--its largest one-day drop since the company went public about a year ago--in the wake of Google's Ad Planner announcement on Tuesday. The Web traffic and audience reporting firm's CEO Magid Abraham went on the record with finance types on Thursday morning, and said comScore would eventually give its proprietary Plan Metrix data to agencies for free, if it felt like Google's new offering was really cutting into its bottom line.

But Elkin said that he didn't think Google was out to muscle its way to the top of the audience measurement heap. "I doubt that it's the intention behind releasing this product," he said. "It's possible that it could be an outcome, but the thing about Google, relative to the other companies, is that they're still relatively new. So all the tools they release have a lot of catching up to do."

This news story is important. I recommend that everyone read it.