I've talked to a guy that has made mods for various systems (LED replacement, VGA cable for Gamecube, etc) and asked him about a somehow, maybe, adding a controller to the DS so that we could get true analog with 3D games. I asked him if it could be done through the headphone/power port, especially since the GBA and SP use that port for Gamecube connections (forgive me if I'm way off, I'm at work and in between calls). Anyway, I asked him if a cord could be hacked to allow a Gamecube controller to work.
He said that, unfortunately, it wouldn't really be possible (I will mention he's in the middle of an SNES->GBA cord for the record) unless the controller could be hacked in wirelessly. So I guess in theory, you could somehow probably do this by making the GBA cart a sort of receiver, and THEN using that to hack into the system to use a wavebird OR some sort of proprietary wireless controller. Or there might be a way to hack into the wireless system itself, but I think you'd still need a receiver of some sort.
That was be awesome, though. Playing all the 3D games with a true analog stick. Probably just a pipe dream though.