I know iOS doesn't let you get at the raw wifi device, so I think the way to do it would be to have an iPhone app that just polls the GPS data, then it transmits the data to a laptop via an audio cable. The audio cable would be connected to the iPhone's headphone jack, and to the laptop's microphone jack, and InSSIDer would listen to the microphone and read the GPS data that way. This is the same method that allows people to swipe credit cards with an iPhone add-on that plugs into the headphone jack.