The update and the dance are the same thing. Each of the google data centers are updated with the new index. Because Google load balances with DNS, and because it takes some time to update all of the data centers, search results fluctuate during this time, hence the 'dance'.
PR is most likely calculated during the period between the deep crawl and the start of the dance. We are in this period now.
Msg#: 12449 posted 11:54 pm on May 3, 2003 (gmt 0)
freshBot: 64.68.82.* bah, listed for only a few days, short dinner date then it dumps you. deepcrawler: 216.239.46.* Good bot, it likes you, listed until death do you depart. So don't make him mad or it will divorce your site. You can divorce the Googlebot by using your robot.txt file. It's much cheaper and faster than going to the courts.