No google don't update Pr, but yeah they always like to update Search results very often , So thats why you see some changes in SERP Ranking
Yesterday I updated the content on one of my sites and optimized the keyword usage a bit and it resulted in a climb to the first page position 8, it had been on page two position 6. Keeping my fingers crossed that it sticks
It's possible that a PR update is on the way. Google Webmaster is currently showing "no data available; check back soon" message for PR in the Crawl Stats page.