They have a lot of incoming links (due to their old content, before they shifted to PPC), and also a HUGE database of 200,000 double opt-in emails, which contribute significantly to the value of the deal. Not a pure domain deal, but obviously the domain is worth at least $8 million of the $20 million, in my opinion.
See the company's press release, at:
http://www.xworks.com/press.asp?y=2005&id=197