I use Prompt by Panic http://www.panic.com/prompt/ on my iPhone to do SSH sessions. Since it was hard for me to type in entries to my Ledger data file I put together a script with Python that could have stored merchants and just sort of make the process easier.
I can type in `ql -m sd -t 50` and it will add a full entry for the grocery store that I frequent. The project on Github is here: https://github.com/robotmachine/QuickLedger
To clarify, it is just a command line tool that I made with mobile SSH in mind-- not a mobile app. Hope that helps.
Pythonista would make possible to run QuickLedger on the mobile, and it could be expanded to automatically update the main ledger, hosted on another machine or on some Dropbox-like container.