Google is opening up Adwords, its main revenue source.
They are creating an API for advertisers that will allow them much more control of how their ads are placed in search and, I suspect, contextual placements.
This is interesting on multiple levels.
First, its a good thing when someone opens up parts of their system, like Amazon and eBay have done. It allows the marketplace to work more effectively and should create more value for Google.
Second, its likely that this move will benefit the big advertisers who have the resources to do something with this API over the small guys who don't.
Third, this is opening up the supply side, but not the demand side of the Google platform. As a publisher who uses Adsense, I have been very frustrated with the "black box" that Google uses to place ads on my blog. I would love to be able to control them in some way, but Google has not opted to open up the publisher side.
UPDATE: This post from Searchviews explains really well why this is a big deal for advertisers and their search engine marketing agencies.