I have seen companies buy domain names with goor
PR and good web traffic stats (IIS log generateed in this instance).
A few months after the purchase the
PR dropped and the traffic stopped,
PR has stabilised, the guy selling the site/domain was a smart guy, he builds back links for sites then sells them on, then moves the back links to the next project.....
My advice, dont buy a domain purely because of its apparent
PR/traffic rating, if the name is what you are after thats another story.
Remeber to check the history of the domain via whois.com if its been around for while, use
http://www.archive.org to view any older sites that may have used the domain, you dont want to buy a domain that has had 'bad' content before...