Arch Linux is a nicely stripped down linux distro. Provided you could find or write a nice ncurses interface for scheme, you could probably build such a system on top of Arch.
I use Arch and Musca (a relatively new, very minimal tiling window manager) on an eee pc 1000 HE and it works extremely well.
A moleskine tablet like you mention with an external rollout keyboard would be so ideal. I'm not asking for a lisp-all-the-way-down (well I would like that too) OS but it would be rad if someone rolled a x-less version of ubuntu or what not w/ the exclusive purpose of teaching/learning scheme.