i'd say its going to be a long while before we see any new handheld system from nintendo because they just released the Wii, so right there is a lot of resources going into hardware production. And secondly the DS is still selling like wild fire. At the very least they're going to wait until sales on the DS slow down a bit before they even consider what's going to come next, and I dont see that happening any time soon based on the sales figures that everyone is putting out.
I do wonder though what the next handheld will be, will it be a successor to the DS or a successor to the GBA? I know they said that their original intention for the DS was to be a "third leg" in their hardware division and not be intended to replace the gameboy line. But i think they only said that so they would save face should the DS pull a Virtual Boy and sink like a rock. But that has obviously not happened and the DS has totally strongarmed the SP out of sales, so it makes me wonder if they even will continue to consider themselves as having a third leg of hardware development or simply fuse the DS and Gameboy lines together, and the next handheld will be the successor to both, one in the same?
Only time will tell but this has seriously got me wondering.