Sgt_Kickaxe - 9:39 am on Jul 19, 2010 (gmt 0)
Google compartmentalizes everything. It all runs together but is stored separately so I imagine pagerank is no different. In fact I would be willing to bet that google iterates several different versions of pagerank. Is your sites social media value on fire via Twitter/Facebook (all nofollow links) but besides that you have no inbound links? No problem, influence has its own pagerank.
If more people's browsers visit your site, and report to Google that they visited and stayed a while, your pagerank will likely increase even with zero inbound links(outside social sites using nofollow). Can't test it but I know it's true. Popularity has value that may not equal number/quality of inbound links, Google knows this and it's weighed and measured accordingly.
One other thing to consider, Google serps fluctuate quite a bit on our end but they may not on their end. The various data centers our serps come from likely draw data from a main data center but at different intervals. If that's the case we can't see the current status of our sites webgraphs, at least not in real time.