I think the reason why Nintendo considers the DS a third pillar is that if it flops, they will just have to come up with a traditional successor to the GBA SP. It doesn't hurt their market share in the portables arena at all. If it is a success, then the DS will have come to be the successor to the GBA SP. Win-win situation for them. I believe Nintendo wants the DS to be the GBA sucessor, but they are introducing a new concept to portable gaming. They don't want to risk it all on the DS.