I will put an end to this.
Last year, Google officially announced that parked domain names will be deindexed. I assume you are talking about Godaddy Expired Domains since this is where most people get their high PRs. Let me put it for you this way, once a domain name expires it's Nameservers automatically shift to Godaddy's and a parking page appears.
Now when Google crawls the website and finds a parking page, it gets deindexed. PR becomes N/A sometimes too.
However, after you acquire the expired domain name and upload some content (along with robots.txt) -- Your website gets re-indexed on Google and you gain back the PR and everything else the domain had.