Windows Live is making some changes to its live search as part of the Live Search 2.0 launch, which make the algorithm for ad placement more flexible to match the increasingly dynamic market for ads. These changes are based on Quality Ranking, where ads that "score" above the mainline threshold get more prominent placement.
This gives advertisers have more control on where your ads appear, determining if they appear in the mainline, which can be done by increasing either the Click Through rate or the Max Bid. While this is generally better for the advertiser, it is largely dependent on the quality of the ad. So some advertisers will see their ad move in or out of the mainline, depending on their click through rate or max bid, or they may need to bid more in order to maintain prominent placement. Others may see their ads moving into the mainline without needing to bid more, and their CPC may actually increase.