Do Dropped Domains Really Retain PR?

Any domain you bought 6 weeks ago that was not in the index would not have shown PR at the last update which was based in large part on data that was 3-4 months old. If the sites were reindexed then you have nothing to worry about. In some cases where the links to the domains are really old (and page unchanged)it might help to get them recrawled.

and yes. the days of sticking a few high quality links to a site and seeing instant results are going away. I have seen it take up to 6 weeks to start seeing changes. In others a week but there is no guarantee anymore than Google will do anything quick.

Domains dropping means a little but not any great deal. Thats a myth that was so fiercely propagated I used to believe it. Whenever Google does do something about the domain aftermarket it will be on whether the name has fallen out of the index. Plenty of legit bloggers have missed that their domains have expired for weeks, refused to pay the high redemption fees at their registrar and then reacquired them when they dropped. As long as they did not yet fall out of the index they held PR too. If they fell out then eventually the Pr comes back. Google has no system at present that penalizes sites just for dropping

Have you kept the original content on them? I've personally watched several marketers pick up dropped domains with high PRs only to see the PRs dive with the next update. In every single case, the new owners were buying for the domain name only, and completely erased the site's old content and added new content.

I've had something similar (though not as dramatic) happen to some of my own sites when I stopped adding content & let them get stale.