For some reason, I've imagined Ripred having a Nick Nolte sort of voice...
Gregor would almost certainly have to be played by an unknown, as Josh Hutcherson (who would've been my first pick) is already growing up too fast.
With all the children's/YA fantasy films being made these days, one would think they'd begin to look beyond UK fantasy for source material. The Underland Chronicles would make -great- movies!