Just turn on network sharing over wifi and run a packet sniffer (in osx). then connect to the new network from iphone over wifi.
SystemPreferences->Sharing->InternetSharing
Cocoa Packet Analyzer http://www.tastycocoabytes.com/cpa/index.php
SystemPreferences->Sharing->InternetSharing
Cocoa Packet Analyzer http://www.tastycocoabytes.com/cpa/index.php