Oddly enough out of all the emulation software I've seen, I've yet to find a decent Nintendo DS emulator that works properly. There are many that work under certain circumstances, but none work for all games. That said, it is hard to emulate a touch screen without using a touch screen (the mouse can only do so much). That said, I've yet to see a game that requires a stylus to play, they usually just make the game easier (that said I have only played a half dozen DS games).
I think that this may be an important aspect of the DS. It is hard to emulate and therefore, people have to buy the games or at least rent them, in order to play. Instead of getting games for free and having their copyrights just ignored, Nintendo can effectively make their games be bought, provided they continue making quality titles.
Just some thoughts. It may just be a matter of time, but by the time the DS emulators work, there may be some new gameboy out thats emulation technology will be too different for the current ones to work.