On 18th February 2009 Google made 'AdSense for Domains' available globally. It seems that no formal announcement has been made, but this has been talked about in blogsphere. Moreover, I can also see 'AdSense for Domains' in my Google AdSense Account.
Availability of 'AdSense for Domains' means that now you will be able to utilize all your unused domains by showing google ads on them. It means you can earn money even on your unused domains. Hosting ads on unused domains is generally known as domain parking. There were few others services before like Sedo.com, but I am sure people will be more comfortable with Google. Also reach of Google makes is available to all.
Though I am not a domainer, I own many domains. Some of the domains were parked with Sedo and now I am moving them to Google AdSense for Domains. Sedo payout was not at all good, so hope it will be better with Google like Google's other AdSense products. I have already moved few domains today and earned around 10 cents as of now - not bad - money for a tea I guess :-).
One important feature that I have not fount yet is the reporting on individual domains. In my account, I see aggregated data for the whole day for all the domains, but I can not see which domain is performing better and which one not. It may take some time to generate that report, so I am just waiting and hoping to see that.