The DSi will have its own store, like the Wii Shop Channel. Now it's highly likely that the old-school Pokémon games will be available.
With this in mind, I can see Pokémon Stadium being released on the Wii Virtual Console. Since the DSi will be able to connect to the Wii, the DSi turns into the Transfer Pak.
Now I know this is just speculation, but just thought I'd throw the thought to you guys!