Google's latest trick is to ban a site and leave its pr showing.I have seen some pr7 sites banned but still show the pr.
To see if you have a ban or not type
link:
http://www.yourdomain.com into google.If it says
Your search - link:
http://www.yourdomain.com/ - did not match any documents then you have been banned.
If it still shows backlinks then you are just penalised or you no longer rate for your kws.
If you buy links from sites for their PR beware of this as banned sites will not pass PR