Thanks for your replies...
Ok, so it seems that for the work I do, and the way I work, muliprocessing in AE is not the way to go ( I do a lot of character animation where i'm constantly previewing 5-10 second areas).
Musings:
My rough understanding of MP in AE is that it's basically launching instances of itself, and so has to save the file, launch these processes and then distribute the data to these copies. Thus the long delay before rendering can start. Is this process just a function of AE's legacy as a program that was created waaay before multiprocessing in desktops or is this just the nature of the beast? It seems like this isn't the case with other multiprocessor enabled apps I've used (3ds Max, Maya)? Is there any way to improve/change this short of a core rewrite of the application (which I don't see happening!)?
mm