1. Get Fiddler
Download Fiddler
2. Set-up Fiddler
Start Fiddler then open the Tools > Fiddler Options...
window

and in the Connections
tab, ensure:
"Allow remote computers to connect"
is checked.
"Act as system proxy on startup
" is checked.
- Note or change what port is set (eg.
8888
).
Once you've saved those settings you need to
stop and
re-start Fiddler.
3. Ensure Fiddler is 'listening'
Once Fiddler has re-started, verify that the
Capture Traffic
menuitem is ticked.
4. Check the 'listening' IP
You need to know your computer's wireless-network IP address to configure the iPhone. This screenshots shows the
Command Prompt > ipconfig
output:
5. Set-up iPhone Settings
With the computer IP address and Fiddler port, go to your iPhone's
Wifi Settings
and scroll down to the
HTTP Proxy
, choose
Manual
and input the Fiddler proxy info:

(remember to switch back to
Off when you're done)
6. 'sniff' away
If everything has been setup right, anything you do in
Safari or other internet based applications will be logged in the Fiddler window.
It's extremely useful for testing/debugging - have fun!
Don't forget to UNDO the iPhone settings when you're finished!!