I think at some point there won't be Nintendo portables at all. Eventually it will be possible to just make one system that is powerful enough to be a home console and small enough to be a portable. You would be able to hook it up to a TV and hook up extra controllers at home and then while on the go you would be limited to wireless multiplayer. So at that point the Gameboy name would probably disappear. Of course that will be a long time from now when games truly reach a point where upping the specs won't make a notable difference.
Nintendo really thinks the DS and GBA will co-exist and the DS is the third pillar. It's not going to be accepted as such and the GBA will be dead by this time next year. But they truly think this is something else so I imagine that someday they'll release a new Gameboy.