Yes some websites and experiencing constant PR updates, even when there are no official statements about Google PR updating. I guess it is much better these days not losing sleep over PageRank that much, because nearly everyone agrees with this fact that it is possible to have a top ranking website, even if its homepage PR score is very low.
By studying some blogs posts, I have almost learned that even Google is not paying that much attention to PR of websites, and therefore not updating the toolbar data very quickly as in the past.
I have to say that I believed in the 3 Month cycles as well. However lately it isn't that easy to say any more. We had Speed Update, Panda Update, +! Update, there are much more things happening that we don't know. I just can say that my Redhat Monitors Page lost quite some Authority since November. The homepage is still going strong, but certain pages don't seem to get the same amount of Google love as before.
its upgrade 3 months once. When is the last the the PR change?
But they do some people told me that Google PR change very often.
This topic been argue in other forum but no one give the real answer. I think here too.