Google has pulled out of a planned advertising pact with Yahoo citing concerns from anti-trust regulators and advertisers. Calling off the partnership, which was announced in June, could open the door for Microsoft to take another look at Yahoo's search business.

That possibility has already been floated by Mary-Jo Foley, who wonders if the Yahoo execs are now "desperate enough" to shed the search business. The Wall Street Journal Marketbeat blog chimes in, calling the failed agreement "great news for ... Microsoft." 

David Drummond, senior vice president, corporate development and chief legal officer at Google, explained the decision in a blog post.

"In June we announced an advertising agreement with Yahoo! that gave Yahoo! the option of using Google to provide ads on its websites (and its publisher partners' sites) in the U.S. and Canada. At the same time, both companies agreed to delay implementation of the agreement to give regulators the chance to review it. While this wasn't legally necessary, we thought it was the right thing to do because Google and Yahoo! have been successful in online advertising and we realized that any cooperation between us would attract attention.

We feel that the agreement would have been good for publishers, advertisers, and users -- as well, of course, for Yahoo! and Google. Why? Because it would have allowed Yahoo! (and its existing publisher partners) to show more relevant ads for queries that currently generate few or no advertisements. Better ads are more useful for users, more efficient for advertisers, and more valuable for publishers.

However, after four months of review, including discussions of various possible changes to the agreement, it's clear that government regulators and some advertisers continue to have concerns about the agreement. Pressing ahead risked not only a protracted legal battle but also damage to relationships with valued partners. That wouldn't have been in the long-term interests of Google or our users, so we have decided to end the agreement.

We're of course disappointed that this deal won't be moving ahead. But we're not going to let the prospect of a lengthy legal battle distract us from our core mission. That would be like trying to drive down the road of innovation with the parking brake on. Google's continued success depends on staying focused on what we do best: creating useful products for our users and partners."