Here's how I understand it.

White PR toolbar - PR was somewhere between 0 and 1 the last time that Google exported PR data to the toolbar.

Grey PR toolbar - PR is not yet exported to the toolbar. If the url is indexed, then in Google's back end, some real PR is assigned, but toolbar PR data was not yet exported.

White bars also include pages caught between updates.

If the toolbar is updated Aug 1st, it could be that no pages added after June 25 show PR, but if Google has crawled them they will get a white bar instead of a grey one.

So there are two types of white bars, ones that are genuinely less than PR1, and those that are the exact equivalent of grey bars.

Gray bar can also mean google has "indexed" the page, but has choosen not to include it in either the main search or supplemental index.

Just because google has spidered a page, it doesn't mean it qualifies for the main or sup.

Often an [ outcast page ] that wasn't in the main or sup will suddenly appear with a very very old cache date.

Google probably doesn't waste the resources to caculate pr on these outcast pages.

thanks for the responses gang!