Tool to monitor HTTP traffic

I use Charles for HTTP monitoring. It can hook into the standard OS X proxy settings (as used by Safari) so it should work fine with the iPhone emulator in the SDK.

To directly monitor the traffic you can use tools like Wireshark or tcpdump.

If you want to send 'fake' requests to a server or have a simple listening server running to receive requests from the iPhone you can use netcat and cURL. I've explained that case in a tutorial if that is sufficient for what you need. If not I recommend Wireshark, which has a GUI and is quite simple to understand.