Didn't Hiroshi Yamauchi say something about DS before it was released, that if it succeeds, it's heaven for them, but if not, they don't know what to do?
Anyway, if i recall, Nintendo announced sometime before DS, that they are going to release 5 consoles (within few years), 3 portable and 2 home consoles. It's possible, that GBA SP was one of them, but DS and GBA micro has been released after that. There have been a "plan B" for the Wii, so that's propably the second home console, N was talking about. But since Wii sells so good, we are not going to see that other console. But, theres a possibility that Nintendo has somekind of gaming machine under development, which combines portability and home consoles. Just think how Nintendo likes connectibility. You could link GBA and GC (Four Swords is one of the best examples), Wii and DS can link via Wi-Fi... So i see it very possible, that they could release a gaming device, that can be as well portable and home console (there was even this kind of rumor about Wii before its release).
@HappySqurriel: I would like to see cartriges back, but unfortunately discs are so much cheaper to make, that it won't propably happen (unless extremely small cartrige, with massive storage is released). DS does well with cartriges, but it's because it's so popular and Sony chose discs, for PSP, that are pretty expensive.