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After last 'update' there are all kinds of problems with Google's database. There are lots of threads about these problems at this forum.
It is unusual for a page to have PR without being indexed. GoogleGuy gave some examples of reasons why this nevertheless could occur in message 8 of this [webmasterworld.com] thread.
In practice, there's lots of reasons that Google might not have the content of the page. There could be a robots.txt file, or the server could have been down, or we might have seen references to that page but not crawled it, or there could have been redirects, meta tags, etc.
He forgot to mention: a problem at Google...