How to securely delete individual files (documents) on the iPhone?
This app in Cydia seems like it would do the trick (provided it works as advertised):

http://cydia.saurik.com/package/iwipe/

The app works by making a very large file that takes up all the free space in the user memory and writes zeros to all the bits. So to securely delete individual files, one would simply (insecurely) delete them (marking them as free space), and then wipe free space with the above app.