iSSH & VIM should be pretty sound. I don't know about Emacs, there may be more difficulties since it Vim doesn't use the command and meta keys so much. This guy does all his coding from Vim in iSSH:
I understand your point about the small market, but if the target audience is developers, then I think that VIM or Emacs compatible keybindings (if only a small subset for movement and selection) are an attractive feature. On the Mac, SublimeText and AppCode, both beloved editors, feature Vim Bindings. And even some of the Cloud Editors (like Ace or Cloud9) have Vim bindings.
I hope it's just a matter of time until the first iPad editor with bindings comes out - and I'll buy right away :)
I'm gonna guess that people using VIM/Emacs is so small a part of the market that apple told us to go pound sand.
Hell, even running emacs across iSSH to a linux server is completely screwed up: Bluetooth keyboards put out multiple key sequences through the iPad.
Please correct me if my info is outdated. Pretty-please. Esp because I would love emacs on the iPad/bluetooth keyboard even if through ssh.