Yeah, PR has lost it's initial value it used to hold. It also has no effect on rankings in other SEs like Yahoo and MSN.
To try and answer the original question, my smart ass comments aside, I think Yahoo's rank thing is based off of links as well. I could be wrong on that...I've honestly never checked my Yahoo rank.
Thanks for the link. I use the Quirk searchstatus toolbar and it doesn't do Yahoo PR. From checking a few of my sites none seem to have a Yahoo PR but two are ranking real well on Yahoo and one is for a competative phrase. I guess Yahoo PR is the same as Google's, doesn't mean much.
The yahoo rank has been broken for around a year. Put in some major sites and it will become apparent quickly