It's crucial that there be a simple and reliable way of securing whatever "stacks" get created. Here are a few ideas (caveat: I'm not a mfg engineer, etc., etc.):

Deliberately including space-filling standoffs *on the gumstix board* more or less on the opposite side of the board to board connectors would help with robustness. They'd have to line up out of the way of parts on the adjacent boards, of course. The idea here is that you just put the whole stacked assembly under a few ounces of compression and you're good to go -- this fails with the current configuration unless users gimmick up their own standoffs or use hotmelt or such. These standoffs don't have to be threaded, just lumps of stuff the right height.

If you could be sure of manufacturing tolerances, the gumstick sized board could slip-fit into a "cage" of (say) surrounding #2 hardware standoffs that bolt through holes in the accessory board(s) or chassis, thus mitigating any wasted space on the "mother" board for mounting holes.

A standard hole form and placement factor, so all the "outsize" accessory/connection boards line up with one another, also seems like a good idea.

Whatever steps you can take to minimize forces on the stacking connectors when communications and power connectors are wiggled is a good thing (this is mom-and-apple-pie, I know).

I think the reset button is a good thing. If you eliminate it, give us some gold or solder pads for the same function (that will still eat up board space, but you save a buck on your end).

      Michael M. Butler  :  m m b u t l e r  ( a t )  g m a i l . c o m
Churchill once said, "When you're going through hell, KEEP GOING".