If you have a jailbroken iPhone, you can move files onto the iPhone for use in the Kindle reader. The reader stores data in /User/Applications/[App ID]/Documents/eBooks. You’ll need to figure out the correct App ID for the Kindle app on your phone (you can probably figure it out by looking at the date associated with the directory).

I have tested this with an unsecured .prc file (similar to what I have loaded onto my Kindle via USB) and the reader opens the document just fine. You may need to relaunch the application to get it to update the available books on the Home screen. Whispernet syncing will obviously not work.