Is it worth registering dropped or expired domains with high PR? If you may the site the same as what it was before, would there be any conflict with PR or making it drop?
It can have it's benefits. I think more important than a dropped domain with PR, is one with PR, alexa, and backlinks. I'm not certain, but I believe that after I domain is dropped, deleted, and re-registered that the PR is reset. But if you can find an expired domain name that has alexa traffic history, PR, and some backlinks it's great because you can benefit from all of the stale traffic that it's receiving. You can redirect it to your sites, or put up a new site with advertisements/adsense.