There is an interesting discussion at Cre8asite forums, I hadn't seen before in a search engine related forum about the number of companies that are buying up expired domains and putting Google adsense ads them. Google has a page for domain parking on their Adsense FAQ. Mike521, a member at Cre8asite, seems to have several examples where Google (or a company owned by google) is buying up expired domains and putting up search portals on the pages. The unique part of this discussion relates to what Google did over a year ago, in that it stopped expired domains from receiving PageRank. The domains were basically reset to the status of a new site. Now I was lucky enough to have bought some good domains years back that got "grandfathered" in so to say before Google initialized this process of reseting domains. It was two domains that eventually became sites later on, but for over a year they continued to hold any backlinks to the site. One site happened to be a blog I did for photography I do. To this day, I am still getting clicks and a few "unimportant" links to the site. Now the value obviously is not the links, but the traffic that comes with the domains for those companies buying the domains. While not a new thing in anyway, it is interesting to observe one of Googles companies doing this. Mike goes on to say "if a domain expires, you'd rather see something there, even just a little search page, instead of a "page not found" error". I'll agree with that.
Continued discussion at Cre8asite forums on Who's Buying Up Expired Domains?