I have just done a review of our AdWords and Overture stats for the last week and compared the impressions. We have made no changes to these campaigns in the study period however Over is showing some "content match" even though it is turned off since their changes last weekend.
Jan. 21 to Jan. 23 - impressions - 16,982
Jan. 28 to Jan. 30 - impressions - 13,912
Down 18.1%
OVERTURE
Jan. 21 to Jan. 23 - impressions - 3169
Jan. 28 to Jan. 30 - impressions - 3653
Up 15.3%
I have 20 web sites that run adsense and all 20 are built the same way. Last week 6 of the biggest dropped off the face of the earth.
The update hammered a lot of innocent sites and I'm sure this is why your hits have lowered by so much.
Simon.