It looks like we can expect a decrease in our AdSense CTR in the future. Google is changing the clickable area in AdSense ads. Instead of being able to click the entire ad block rectangle, you can only click the ad link title (first line of bold text) and URL (last line of small text). This is supposed to reduce the number of erroneous clicks which many websites rely on for revenue.

It sounds like this change isn’t set in stone yet. As an AdSense publisher, I hope Google ultimately decides against it. If Google does decide to go through with the change, hopefully we will see advertisers dumping more money into Google. A larger percentage of legitimate clicks will drive more quality viewers to the advertiser’s website, giving them a reason to bump up their advertising expenses. It could take some time, but it all may level out - our CTR will drop while our PPC revenue will rise.

You can read more on this over at JenSense.