Contextual AdSense Revenue Plunges Why I'm Switching To Yahoo

So I'm about ready to switch back to Yahoo Publisher Network for contextual ads.
Because although my page hits are high, and CPM has dropped -- The revenue has plummeted!
Here's an example:
Monday, March 6, 2006 $7.52
Monday, April 3, 2006 $2.69

Adsense is not something "I need" , it's nice to cover all the costs of hosting, but such a dramatic drop is disturbing.
Has anyone else using AdSense noticed a fall? I'd have thought if there were changes other people would be mentioning it.

I'm still thinking about changing to YPN, they owe me $30 bucks so I need to get to $100 for the payout.

Anyone know of a plugin for movabletype to load balance ads per post? Like say all the even posts get Google AdSense and all the Odd posts get YPN or somesuch?