The following worked for iPhone 4S (iOS 5) and Macbook Pro (10.8.2)
On your Mac, go to System Preferences > Sharing > Internet Sharing
On your iPhone, go to Settings > Wifi and choose your Mac as the Wifi Access Point. Press the blue detail disclosure next to it to and note down the IP Address (192.168.2.2 in my case). At this point, the wifi icon on Mac's your taskbar should change to the following:
Open wireshark. Click on start capture, and use the new bridge interface that should now be available among the options.
???
Profit!
As with all stuff networking related, you might have to restart wifi etc and repeat steps and invoke your favorite deity to get this incantation working :)