The solution is right there. You don't need to buy anything other than the DS. Technically you could use a Wiimote as a pointer in place of the stylus, but having a DS controller is perfect.

Imagine playing your ds game as normal and sitting on the couch. You have the big screen behind or all your gaming goodness but when you really need to do somthign fine a miniature version is tthere to poke at with your stylus. 

AS far as bandwidth issues there is no problem. Just have the ds dump the whole game to the Wii and it only has to send the controller function over Wifi not the image.

 I can totally see them doing this. As the world moves twaords portable they will leverage the DS to promote the Wii. Hence DS download station.

The  only thing that would be a problem is that they may fear that the DS games would hurt Wii game sales.

I am willing to bet that even if this does not happen the next version of the Wii/DS will be heavily interoperable. 

I was actually thinking about that solution myself, but didn't want to say anything until I compared how large of a file size that would take up on the wii.

The only real problem would be looking back and forth between the TV and the DS screen, which I think is why you'd need to have a Wiimote to use in place of using the stylus, since there's no way to show where your stylus is on the TV unless you're touching the bottom screen of the DS with the stylus. If you lift up the stylus, you'd have to keep looking back and forth from teh TV to the stylus to tell where you'd be touching when you put your stylus back down again.

Technically, it should be possible to do the combined setup, have the DS in one hand, and the Wiimote in your other hand. that would give you the microphone and the buttons you'd need in the right place. then you could just have the B or A button to count as tapping the stylus on the touch screen, i think it would even be possible to transmit the music back to the DS lite, so that you could wear headphones to listen while you play.

Of course, that raises another cool possible feature of being able to use your DS as a wireless receiver for music that is beamed from your Wii console.