mankoff

I use emacs on my iPhone and plan to use it on an iPad if I get one. It isn't the official emacs, but is 'mg' the MicroGnuEmacs editor

http://en.wikipedia.org/wiki/Mg_%28editor%29

Jailbreak your iPhone/iPad and Cydia will allow you to install Vim or mg, plus Ruby, Python, etc. MG even supports LaTeX syntax highlighting, although of course the compilation must be done elsewhere.