The new page ranks are a farce! I have a trading blog with extensive information regarding trading and it only has a page rank of 3 after the update. Another single page I have contains a few words about cat sponsorship and is only linked to another page about child sponsorship. It has a page rank of 4. This is a joke! I now know why people are starting to ignore it.
Page Rank has nothing to do with how good the content is or isn't. PR is determined by incoming links to your site. Links to your site "pass some of their PR" to your site. The higher the PR of the site linking to yours, the more ability to pass higher PR. The number outgoing links can also affect how much PR it passes. If you are concerned about a higher PR, get more sites to link to yours. The more relevant, the better.
PR used to be only about inbound links, but I don't think that's entirely accurate anymore. My seo blog went from PR0 to PR5 with this update. It has a total of 3 inbound links from related sites and an rss feed (which only 3 or 4 sites are streaming right now). I can't believe that less than 10 relevant links would give me that much PR.
For some reason this isn't always true. I've had a PR8 link for 3 months straight to my PR0 domain. Guess what it became after the update? PR2, that's a damn joke if you ask me.
I repeat, PR is based only on incoming links. Non-relevant links do count for PR, they just aren't as important for SERP's. Also, google only shows a portion of incoming links as backlinks, but (1) link is all it would take to get a PR5 as long as it was a high enough PR link. I have a PR5 site with very few incoming links - and others with a lot of incoming links. I've found it pretty easy to get a PR5, while a PR6 or higher remains very hard to obtain. Try doing a search on google with quotation marks "www.yoursite.com" and also on Yahoo and MSN. I'll bet you see a lot more links than google is showing as backlinks.
It isn't as simple as looking at the PR of a link. The number of outbound links devalue the PR it passes to your site. A link from a PR8 site with only 5 outgoing links is much better than one with 50. A PR8 with a lot of outgoing links may not be passing very much PR. You have to be careful about the links you have pointing to your site. There is an exploit in the google toolbar that allows you see a bogus PR, so some shady people will sell you a high PR link, when the page has none. There is also a way to prevent a site from passing PR- and google also prevents some sites from passing PR - like link farms known to be selling links. If you purchased this PR8 link, you should take a closer look at it. It sounds like something isn't right.
I had one site move from PR 3 to 5, one site move from 0 to 4 and another site move from 0 to 2. First one' SERPS did not change, other two are still in the sandbox. All other sites PR has remained the same.
The benefit of backlinks is ongoing. The toolbar update won't affect your SERP's because the links have already been factored in. Your PR5 site was being counted as such before it showed up on the toolbar update.