We have also been tracking our Adsense data over four years, but we see very little consistency within the data set. I'd say it's almost random data. However, the fog clears up once we start to look at longer periods, e.g. 90 day/200 day moving averages. THEN data becomes quite predictable.
Anyway, I see =exactly= the same trend as you. Beginning in September, eCPM and CTR have been dropping seriously. The trend is rather scary. If it continues to drop like this, I'll be starting to =pay= Google for running ads by April/May timeframe.
Possible causes? The economy, mostly. If consumers are worried about their jobs and paying back mortgages, they change their spending behaviour. And they change their behaviour on the net (looking less for luxury goods and services, looking more for price comparison and saving potential).
It is not unrealistic to think that this behaviour change may affect ROI for campaigns on the content network, and that advertisers stop using Adwords and the content network, or seriously reducing their spendings.
I guess it will all clear up once Google reports earnings for Q4/2008. If they post another record quarter we will probably be pulling Adsense from our sites and focus on affiliate marketing programs (because then they probably have changed the payout to small publishers to increase their earnings which is not OK). Should they admit that the quarter has hurt them as well, we have at least proof for the recession hitting Google.