I have a feeling it'll soon be very, very easy to pirate, as long as you're within a WiFi access point connected to a computer that hosts the ROM dumps.
Since the DS can download and execute programs wirelessly, it's only a matter of (short) time before someone figures out how to do that with their own programs and commercial programs. I don't think it's out of the realm of possibility that someone eventually makes a PC program where you can wirelessly stream a game as it's played on the DS, especially games that are not programmed to be sensitive to the delay.
It'll basically be the equivalent of playing a GBA game on your PC: it's not really portable, but you can play any DS game you want.
More immediately, I think you'll see home-made programs and cut-down commercial games (to fit in the RAM of the DS until someone figures out a streaming method).