Simple supply and demand.  TechCrunch reports that search query volume growth has slowed down rapidly and is now at 7%.  As you can imagine, spend on search is growing much quicker.  The result?  Higher CPC (without necessarily higher quality/better traffic.)  This trend will continue until each vertical reaches a ‘market clearing’ price for what a search visitor is worth.  Net-net though, it makes it clear that focus will continue to shift to display given that market has the opposite supply/demand trends.