I too experienced a drop in the adsense earnings from the second half of December 2009 till now with traffic fairly remaining the same during January 2010. May be adwords people are reluctant to spend more or the visitors have become conscious not to click ads??
However, I think it is time to Google allow us to use new ad formats and maybe to add images in textual ads (like Chitika ads)... Did you know that 85% of all clicks on ads are done by just 15% of surfers? Almost everybody today know that an ad is an ad and will not click it... It is time to make some changes which will result with higher CTR.