Since the hardware capabilities will all be very similar, and since most everyone already has a DVD player, and since GCN2 will be cheaper, I think they have a real chance to take a bigger share. Maybe 30%. Sony will probably be 40-50% just on hype alone and NextBox will be 20-30% ish.
It will all come down to launch games. You know M$ will have at least one big one maybe Halo III or something. Sony will have nothing exciting. And it just depends on what Nintendo puts out there. If they launch with a good Mario and Pokemon game, plus a good group game like SSBM2 or Mario Party 6, then follow up 4-6 months later with a Zelda and Metroid game they will do great. If they launch with Luigi's mansion II and no other games for a year, they are dead. I all comes down to games, Nintendo makes the best games. There is no doubt that they will produce great games, the only variable is when will they come out.