Very neat concept. Modular external computing components. I see you've mixed PCI-X and PCI Express. I would think you'd want PCI Express connections between expansion components, and reserve PCI-X connections for within a single (of course optional) box to support cards of that sort (if desired). Issues I see are the following: popup and hidden storage will require space that's hard to come by right now. Also, configuration of a multiple box system will undoubtably be more expensive than a single do-it-all unit. But you do have some superb expandability!

I like your idea for a place to store the remote. Apple should've done something like that on the MacBook. HP has a place to store the remote on one of their notebook. It looks like it's spring-loaded or something.

The dual X16 connections on the bootom of each unit are there to provide a high bandwidth connection between modules. The full bandwith needent be used, just available. Apple could provide barebone DIY modules for user to add their own peripherals at a minimal cost and provide custom modules that are plug and play for those who are skitish about mucking around inside thier box. As for the hidden compartments a slight increase in case size would be necessary, minimal impact if added to the hight instead of the footprint.