I had the same idea a couple years ago. It started out as an idea to dock my Zaurus SL-5500 (or was this on my Vizor Prism?) to something that could give it video out, then it quickly changed to just keeping my home directory synced to my Zaurus' SD card. Now years later, I realize that all I really wanted was to be able to get at my files from anywhere in the world. What I realized is that it's almost always cheaper to have two specialized devices with access to each other's storage, than to have a core device and some kind of dock. Not only is it cheaper, it's also faster and more flexible.

What I really need is separate devices that each have high bandwidth, 24/7 connectivity. That connectivity might be between them or to a centralized server.

@GeneralAntilles: CPUs and RAM could never get enough bandwidth and low enough latency through an expansion port, but graphics are a different matter. External graphics cards connected over the express card slot have been demoed but I don't know if anything ever came to market.
http://www.engadget.com/2007/01/08/h...-external-gpu/

-John