When a domain expires it will most likely loose all (or most of) it's inbound links since people doesn't want to link to a non-existing site, and it will then loose it's PR after one or two PR updates...
Google will also notice, when googlebot tries to crawl the site, that the domain isn't regged and google will remove it from it's database.
Then it will get no PR at all....
This can take a while (one or two updates), that's why some backordered/snapped names keeps their PR long after expiration.
It's real easy to make a fake PR for a domain name (it's even possible to get a PR 8 or 9), that's why I think that PR's not important for the domains value.
Unless it has a lot of backlinks....
Without the backlinks the PR is useless.
You buy a high PR domain name, make a site, and you are happy with your PR 6 or 7, but after the next one or two updates.....
Poof, your precious PR is gone (unless you have the backlinks)..
