^For practical purposes I think they'll probably be casting midgets (are we still allowed to use that word?) to play the dwarf roles. I can't see a TV budget being able to deal with the kind of in-camera forced perspective or post production compositing that would allow someone of average height to appear to be under 4'.
With that in mind, whomever they cast as Cherry would probably be an unknown actress. Either way, I can't help but wonder if they'll chicken out and loose the beard.
To my mind, Carrot and Angua are the real challenge. It's not enough to cast people that look the part, they really need to embody some really rather complex and subtle characteristics.
The trolls might be a little bit tricky too. I mean I'm sure they can build a costume, but getting it to emote might be tricky.