This sounds cool. If I'm understanding you correctly, this would allow stacking both vertically and horizontally. The horizontal just meaning, "not on the current stack".

Why not split the two functions up? This is all about prototyping or one-off installs. You could have an RTC shield and a separate display driver shield... would that free up room for the display header?

Sounds great... I'm wondering if some of our ideas may not be completely confined by the size if we used components off the form factor. For example anything requiring a heatsink could have a header off the board to be mounted as part of the enclosure. We still get the 'quarter stack' for the main logic and I/O and have the ability to place MOSFETs and the such as part of our enclosure.

A HopeRF wireless shield using the following would enable us to put together really small wireless sensor packages: http://jeelabs.com/products/rfm12b-433-mhz !