Great slides on the Actor Model by Steve Dekorte of Io fame
Steve Dekorte, C4 2006: Actors, a Unifying Pattern for Scalable Concurrency
You've probably noticed all the del.icio.us links to Actor Model and programming language stuff. If you want a good summary, I think these slides are great. If you aren't too interested in the Io language stuff, you can gloss over those slides, you'll find "big picture" talk before and after that section.
This stuff is going to be my life for the foreseeable future.