The PR toolbar (which is what us plebs see) isn't updated very frequently or consistently - six months, nine months, a year. How often Google internally updates PR - who knows? Probably more frequently, but the SERPs/algorithms are of more interest to them so maybe not. PR was always generally more for our benefit than theirs.
theonemanyuri - It's interesting because often I see that sites go down after solid link building then pop up again higher a while later. The link builder often assumes that it's because of a dodgy link or over linking, but I'm wondering whether that's really the case. We panic and stop link building and the site returns, we assume it's because we've stopped but maybe it's because the links have had time to be evaluated and 'settle'.
I started a site about a month ago which still has PR: unranked because there's been no toolbar update, but I've only been submitting it to low PR directories - about 15 so far- and its holding up well in the SERPs with surprisingly little bouncing around.