I have bought a lot of expired domains, and even some dropped ones, to boost the PageRank of my network.
Sometimes I 301 these to my existing sites and sometimes I link them to my existing sites.
It is maddeningly difficult to tell if these domains still possess "real" PageRank (as opposed to published toolbar pagerank) and are still able to pass that PageRank along to my other sites.
Non-scientific evidence seems to strongly support the theory that at least some of these domains still have juice and are able to pass it along.
I haven't yet devised and executed a test which can reliably prove this one way or the other.