The domain “Adsense.com” was owned by AdSense Consulting, who bought the domain in 1996. And they’re still kind of bitter about it. Go to AdSense2.com and you’ll find a sharp-tongued message:
If you think you can get rich quick placing other people’s ads on your site or blog, please contact Google who has taken and used our business name without permission or compensation. At least they’re clear about their feelings on the matter.
Google didn't’t own AdSense.com until as late as July 2006. In July, Wired reported that AdSense Consulting had sold the domain through a private broker, the buyer taking control of the domain in September ‘05. But as of the date of that article Google still didn't’t own it.
A forum poster at Domain State noted that Google finally acquired the AdSense.com domain within the past week. It is unclear if Google bought the domain in 2005, or if Google bought it last week from the aforementioned broker.
The Whois Registry shows the record was last updated on October 31st, perhaps an indicator of when Google officially registered it. AdSense.com now redirects to the Google homepage.