I think google has made it more difficult to achieve a high page rank because many people have been manipulating their page rank over the past few years.
I had a pr 6 a few months ago, stoped working in that site and it's now pr2. Same inbound links but no new fresh links. Fresh links are the most important thing in my opinion, it doesn't matter if you have hundreds of backlinks, you need to get new ones all the time.
My analysis shows that Google has made manual updates to the toolbar PR of individual sites. I have found about an equal amount of increases as decreases. In my case I had a bunch of sites go from 0 to 3 or 0 to 2. These sites have been ranking well all along. I have recently made steps to improve my compliance with Google's guidelines and submitted a re-inclusion request. I guess "Resistance is futile".