A new version of Textastic was released today for both iPhone and iPad, and among the new features are iCloud syncing and some fairly deep support for the
x-callback-url specification. Creating, opening, and appending to files are all supported actions, and a healthy list of parameters allow for all sorts of potential manipulations.
One especially cool usage is detailed at the bottom of the API documentation; it makes it easy to grab a Web page’s source HTML from within Safari–and I suppose it could also be used to receive a server’s error responses for debugging purposes. I can definitely see myself getting a lot of use out of that one feature alone.