The_Shower_Scene - 1:07 am on Jan 11, 2011 (gmt 0)
Toolbar pagerank updates daily, but only for a small percentage of sites.
Toolbar Pagerank still updates more or less quarterly. You can confirm this by listening to Matt Cutts spell it out here [youtube.com] in a recent video.
For sites that see their Toolbar PR chagne outside of the regular updates, if the Toolbar Pagerank turns off or suddenly turns on then that's more likely the result of a site coming in or out of a penalty (either automated or manual), which may or may not affect traffic.
This in contrast to how Toolbar Pagerank used to be updated batch style quarterly.
It's still updated more or less quarterly, as explained in the above video.