Terminal IDE and AIDE for Java. SL4A for Python, JavaScript and Ruby. Ruboto. There's a version of Octave now. And I'm sure there's more besides. There are definitely development options on an Android device.
I've been using SL4A and the Jota editor to teach myself Python (professionally, I'm a .Net developer) on my Nexus 7 and I've been pretty happy with it. I can carry it everywhere, and anytime I have a few minutes to spare I can open up the ebook I've been reading, then go to the editor and/or the immediate window and try some things out. And if I'm at my desk, I can use a hardware keyboard, making things even easier.
Emacs improves the possibilities even more. At the very least, it's a far better editor than any of the current editors (unless you're a vim fan using Terminal IDE) for any scenario where you need an editor.
At best, it could get a whole package management system of supporting utilities that are available to it inside its terminal, and becomes a good way to develop with virtually any language that has a compiler that can be ported to Android. Maybe it even gets w3m browser mode, music player mode, and other awesome Emacs-as-OS features.
I've lost the count of how many times I have closed a browser window while trying to cut text.
Unless otherwise approved by Apple in writing, no interpreted code may be downloaded or used in an Application except for code that is interpreted and run by Apple’s Documented APIs and built-in interpreter(s). Notwithstanding the foregoing, with Apple’s prior written consent, an Application may use embedded interpreted code in a limited way if such use is solely for providing minor features or functionality that are consistent with the intended and advertised purpose of the Application.
That would certainly exclude Emacs.
