tedster, you said "...I can definitely report success [in moving sites]... By success, I mean no traffic loss at all in some cases, or only a few day's disruption." and also "PageRank still does matter".
I agree that PageRank matters, but if I am moving a part of the website on a new domain, generally the old website has some PR, but the new domain has no PR (it's a new domain). So, I can't understand what did you do, in order not to have traffic loss (from Google).
Did your new domain already had a good PR? If so, how did it achieved it? Did you created new content on the new domain a few month before the movement and promoted it, in order to give it its own importance/traffic?