If you can deal with the lag, showdown already works on iphone/android
As for an app... Well, there might be legal ramifications, since Nintendo has made itself clear we are not to profit from a simulator. Hosting it on itunes any other way would be a pretty hefty deficit, and that's not mentioning getting people to program it... In all, probably not going to happen, sorry.
That said, if someone were to prove me wrong, i'm right in line behind you lol