vi comes with (at least Cydia's install of) Terminal.app. The point of an iPad IDE, though, would be to take advantage of the iPad's touch-based workflow; if you're just going to type, you may as well use a PC. I want to finger-paint my programs into existence.
It seems to me the main problem is then the keyboard. As far as I can tell, there's no way I can run Emacs with the iPad soft keyboard (brother in law bought one, so I got to try first hand). But I just have a clunky laptop if I attach an external keyboard.
This is also the problem with the ssh client on my G1. I can do minor tasks in the shell but I can't use my editor.
