Google blocked Microsoft out of AOL by taking a 5% stake in the company for $1 billion.

The deal involves also a $300 million worth of free Google advertising for AOL.
The bigger topic though is how far AOL got Google in displaying graphical banners on their sites. Google already offered Webmasters the option to display graphical ads through the traditionally text based Adsense since a while.
Banners on Google search results would disappoint tons of loyal Google users. I do not think they go that far. We will see. It could mean tons of money for Google.
Update: Google will not have graphical banners on their own search result pages. More details on the Google AOL deal in this Reuters story.
BusinessWeek has just published a good article about the deal arguing that Google gains the most out of this arrangement.

See also the official Google press-release.