I think google follows the following rule.. They see the rate at which a website is increasing the number of links. I have seen many websites having a lots of good quality backlinks have been dumped from say PR6 to PR4 or so.this is probably because of the constant number of links.
Or, those backlinks lost PR themselves for buying\selling text links, which in effect would lower your PR. Beware who you cross link to, if its a one-way backlink there is nothing you can do about it.
IMO. Google stick on their own rules (from the Webmaster Guideline) about paid links. As a result, a lot of site suffered from PR drop as 'punishment'
what will u say for one of my blog? it has PR4 but believe me , i havent posted more than a single post till date.
I think depends on more things, include backlinks, time, but now maybe new site with more backlinks ,easy to get higher pr, but also easy to down pr
This is what I have suspected for a very long time. If you have a quality site, the no. of backlinks should increase naturally over time.