According to gizmodo a new iPhone application is being developed (already working) to enable P2P on the iPhone.  This is exciting news for many but it is still in early stages of development.  A few things to note about this application as of right now.

• This is a command line client right now, which makes it much more difficult for the normal user.  You should wait until a GUI has been created.
• You have to use WiFi for this to work
• Your battery life will be reduced drastically

Link to application