Kotaku caught a GDC presentation by Masato Kuwahara, Nintendo's DSi Hardware Group leader, where he ran through some Game Boy concepts that fell off the fast track at one point or another.
The nearest miss is the dual-card DSi, apparently rejected for being a little too fat. Also shown was a 1998 Game Boy Advance SP touchscreen "concept" which never made it past the "let's bolt a touchscreen on and see what happens" phase.
The most interesting not-quite in the whole presentation is the lard-assed Game Boy Advance predecessor from 1995, which was shelved for terrible performance and, well, the fatness. That's a DS next to it, in case you were wondering. Check out the full story at [Kotaku]