I don't doubt that there are many and varied reasons why webmasters would like to know when the next toolbar PR udpate will be, but: (1) wishing you could know doesn't alter the fact that there is no way you can kn ow, until it happens; and (2) if you are making marketing decisions or site redesign decisions based on speculation about when toolbar PR will next be updated, (a) see my previous post about actual PR vs. toolbar PR, (b) understand that when toolbar is updated you will see no change in Google ranking (SERPs) because that has already happened and is happening on a continuous basis -- PR updates change nothing as far as ranking is concerned, and (c) as fryman said you have a lot to learn about SEO.
Xeno- internal updates which determine your websites weight happen all the time. Publically visable PR rank happens like 4-5 times a year. Its just for vanity or what minnie said _^