Not sure I agree. Modern dev environments are not like the BBC Micro - and we really don't want to impose 80's micros on our kids, do we?
(I do think that a single-purpose boot is better than a windowing environment though.)
So... Scratch? Logo? Anything around Pygame? (Maybe even sugar. Much depends on the target audience and the intention of the teaching. Could be anything from literacy to electronics - doesn't have to be computer science.)
http://www.raspberrypi.org/forum/educat ... plications
Raspberry Pi Tutorial 7 - Object Oriented Game Programming with Python and PyGame [video]
Using turtle graphics