Haven’t had the chance to use PureMVC in a while, although the compensation is that I get to do some cool math at my current gig. When possible, I try to keep up with developments in the PureMVC field. The MultiCore implementation where the core actors are implemented as Multitons, not Singletons is very interesting. Messaging between cores is implemented via a pipes utility.
Here are a few links to get you started in these areas.
HelloPipes tutorial using MultiCore and Flex Modules.
MultiCore app. demo without Flex Modules.
Things to watch for when porting from PureMVC to PureMVC MultiCore.
The Algorithmist 
Another really useful tutorial that I found is Understanding PureMVC MultiCore( http://www.joshuaostrom.com/2008/06/15/understanding-puremvc-pipes/ ) . It took a few reads for me but it really cleared up a lot of the implementation details that were stumping me.