The battle over the search space has been escalated by Microsoft recently. They have made some deft moves acquiring web partners and some talented people and finally have announced they are providing 100% of their PPC traffic.
Microsoft is working hard to build its publisher partners and the addition of Amazon - once a Google stalwart - will help this effort. According to an article by David Utter, the newly unveilled Windows Live Search results are now being served for Alexa and A9 site results.
Add the recent hiring of Ask CEO Steve Berkowitz to their team and the MSN push is starting to look aggressive.
By finally taking over the full PPC traffic distribution they have just created a whole in Yahoo!'s numbers - though it has been a gradual transition allowing Yahoo to get used to the loss.