The official Google AdWords representative at Webmaster World informs that soon enough advertisers will have the option of choosing out from the AdSense for Domains network.
"We are working on making it simpler for advertisers who don't want to show on domain sites or error pages to exclude them. In the meantime, if site exclusion does not seem to be working for you, you can write into support for help."
Rusty Brick says "The AdSense for Domains program has always sparked huge debates in the community and industry. So this step by Google will be a much desired one and an overall great move for advertisers."
So far, Google has not given a date for when advertisers can start opting out.