just found the "Daruino" pages - Google had been not-so-helpfully redirecting me to a search for "Arduino", assuming "Daruino" was a typo. Perhaps a more unique name would improve your searchability?

Your 28-DIP plug-in board seems like a great place to start. It's a useful gizmo in its own right, as an ARM-breadboarding tool, but the "just swap this in to make your Arduino run faster" angle is a great gimmick. I would definitely buy this, if only to play with it.

What scale of manufacturing do you have in mind? If I decide to order fifty and want them in two weeks, is that likely to be possible?