Lets draw a difference between a PR Update and a Backlink Update and a Rolling Update, shall we? It's confusing people.PR Update
- updating the server center that makes the pretty green bar move back and forth. Has an effect on SEO newbie heart rates but little else.BL Update
- updating the server center that makes the pretty link list fill up or empty out. Has an effect on SEO newbie heart rates but little else.Rolling Update
- the big one. Google not only has to check YOUR backlinks, but also those backlinks' backlinks (and so on) in order to make sure you are not in a link farm system - this takes time - usually at least one crawl (4-6 weeks) and more often 2 or 3 crawls. Until then you don't get full benifit of your backlinks.
This is the TRUE backlink and PR update, and it's ongoing, not cyclical.
Previously, Google would "estimate" your PR, relevance and authority for pages and give you a temporary score until it finished this - resulting the the infamous issue where new sites would come out of the gate ranking well and then dropping back after a month or two, thereby causing a panic...
This has been replaced by a system where new sites are no longer given the traditional boost, and actually have to wait until the backlinks have been fully analyzed, thereby causing a panic...