Back in September, Microsoft, Yahoo and AOL held a meeting to discuss a potential advertising partnership, which would see the companies banding together to compete with Google. It appears that we’re about to see the fruits of that partnership.

Today, the three Internet giants announced a partnership officially, which it says will benefit advertisers, agencies, and publishers by providing “more efficient access to premium online ad inventory.”

The agreements among the companies will enable ad networks operated by each to offer one another’s premium non-reserved online display inventory to their respective ad customers.

The companies say that while advertisers can continue to partner across the Yahoo Network Plus, AOL’s Advertising.com and the Microsoft Media Network, each is differentiated by its capabilities around data, optimization, packaging and inventory. The partnership, they say, will enhance demand for and value of each party’s display ad offerings and provide better yield for publishers and advertisers.

“We’re thrilled to partner with Microsoft and AOL and bring to market what we believe will be a more efficient, effective and more effortless way to access true premium inventory and formats,” said Ross Levinsohn, Yahoo EVP of the Americas. “There has a been a significant shift in how inventory is bought and sold, and we’re now 100% focused on controlling our own destiny, working directly with marketers and agencies and driving better returns for our advertising partners. “

“Enhancing choice and scale in today’s display advertising market is a rising tide that lifts all boats,” said Rik van der Kooi, Corporate Vice President of the Microsoft Advertising Business Group. “This partnership will create an opportunity where advertisers and publishers alike can benefit from easier access to – and demand for — high quality inventory. The fact that we’re joining together to offer this kind of access to quality – yet each with our own differentiated ad offerings — is something that will benefit the market as a whole.”

“We are excited to be part of this partnership,” said AOL Chief Revenue Officer Ned Brody. “Today’s announcement sets in motion the opportunity for advertisers to achieve scaled solutions across premium publishers. This should reduce friction in the marketplace which will benefit both advertisers and publishers. And this partnership will take our existing Advertising.com partnerships with both Microsoft and Yahoo! to a new level.”

The companies will continue to compete for ad spend and publisher partners. Early 2012 is the target for integrating one another’s real-time bidding technologies for non-reserved inventory.