One can only hope it's not another revamped gameboy. We have enough of those
They've been going to great lengths to convince people that the DS and GameBoy are different products (some analogy about a third pillar), so we'll probably keep getting new GameBoys. I have to wonder if it isn't diluting their market too much though, especially with some of the other new stuff coming out. PlayStation 3, Xbox Next, PSP, the next cube, GBA, N-Gage and now DS? There comes a point where there's just only so many game systems that can succeed, and I think we're there (even without the N-Gage).
But I could be wrong. Even if half of GBA's buyers get a DS instead, they'll still both be selling relatively well. I think a lot will depend on the kind of support they get; if it ends up being just a gimmick like the GBA-GCN thing, they will have failed, but hopefully developers will be interested enough in the concept to really branch out and think creatively. Only time will tell...