The real value of AdWords is decreasing for its customers.
Why?
Hijacked URLs replaced by phony directories for the sole purpose of generating AdSense revenue result in worthless clicks.
These phony directories are proliferating at an alarming rate, as are the worthless clicks; forcing Google to charge less or lose customers.
This week I am spending half of what I did last week on Adwords. Normally that would call for the champagne but I'm also getting half the impressions, half the clicks and therefore half the sales leads that I was 7 days ago.
Gmorgan, in addition to the other comments here, I'd add that this is the sort of thing that one might see if they were no longer appearing on any partner sites.
Have you made any changes to your ad copy and/or URLs recently? If so, then your as will come down from partner sites until they can be reviewed and approved.
BTW, I'd always suggest contacting AdWords support if you are experiencing something that you simply can't figure out. Usually, a look at the account will net you an answer in short order.
AWA