An iPhone Hacker, Core, has ported a P2P torrent application to iPhone. The software is based on Mac OS based client Transmission.
The application is still very raw, but functional. It has no GUI, though. It is a command-line too. EDGE is still a problem. So you are left with WiFi.
Another problem is that the application will quickly eat up your battery power.
Again, if this application is still very preliminary and must be used only experienced users. If you hardly find the power button on your iPhone, you'd better stay away until the developer polishes the application and it gets a proper GUI and installer support.
For those experienced users here is the link: http://wickedpsyched.net/iphone/torrent