It DID occur about every 3 months, but the most recent toolbar PR update was due at the middle to end of August, and it didn't happen. There could be a lot of reasons for this - a large update that is taking longer than expected; a change in the frequency of updates; a alteration to the toolbar that removes PR entirely. No one can know when Google will update the toolbar PR. Maybe it won't happen ever again...
Keep in mind that Google's internal PageRank information is updated continuously - that is every day as they crawl the web. Its only the public PageRank database that the Google Toolbar uses for its display that is updated periodically.
Indeed public viewable pagerank is all smoke and mirrors. Watching a PR toolbar is like watching grass grow via an irregularly taken photograph.