Is anyone else seeing this?

Nothing noticeable.

I'm not 100p/c convinced, but there again, I love a good conspiracy theory ;)

How long have you had adsense on the sites which are dropping?

Do all your AdSense sites use similar layout or placement of the AdSense code?

If your AdSense code is before your content, and Yahoo suddenly placed more emphasis on how far the content is from the beginning of the file, your AdSense sites would take a hit and your sites without that code would do better. There are lots of other explanations than Yahoo targeting AdSense.