There are actually several solutions for cross-platform on-the-go Dropbox file editing, like Documents To Go Premium (US$14.99) for both iPhone and iPad. However, if your need is just for plain vanilla text file editing for iOS, you might want to take a look at Droptext ($0.99) from developer Kevin Smith.
Any type of file with a MIME type of TEXT can be opened from Dropbox and edited in Droptext. Those files types include .txt, .php, HTML, CSS, and many source code types. Smith promises to add the ability to add your own file extensions in a future version of Droptext. Editable file types appear with a text file icon when you browse your Dropbox, and they're opened immediately with a tap. When you're done editing, tap Save, and the file is saved back on Dropbox in its revised form.
It's not a fancy app, but Droptext does what it is supposed to do -- bring Dropbox text editing to iOS.
[via Wired Gadget Lab]