I'm unable to verify this but I'm suspecting that the safari web inspector can only connect to apps signed with a Developer cert/profile.
Are you building with the PhoneGap Build Service, or locally with PhoneGap? I ask because I thought that you could only upload apps to testflight directly from the XCode project, which would imply you are building locally.
Yes, only developer certs support web inspection by Safari. Distribution certs will prevent Safari from seeing the web view as inspectable.
I should note that there's nothing stopping one from using Weinre. Just be sure to disable that before releasing into the wild! ;-)