You don't need to be in an airport to get on travelnet, so your normal data plan on the iPhone should be just fine. The browser may not be the best, since there probably isn't a mobile/iPhone version of the site, but you should get by, at least.

As for the flight itself, it helps that you're essentially going from one hub to another, with plenty of cities that both airports serve. Take a look at the wikipedia page on your airport of choice (LGA or JFK) and find a city with a decent connection time (you don't need to give yourself too much time in the smaller airports, but watch for delays) and loads if the nonstops aren't very good.

Example: I was in YYZ with a friend of mine, and we were a few hours early at the airport. A flight was boarding to DTW when we got to the gate, and the agent (awesome guy) was able to get us on, exit row seat and everything. When I got to DTW, I realized that I couldn't get home on a nonstop to MSP until 9:00 if I was lucky, since the standby lists were all out the door. Looking at the flight monitors, I noticed a flight to RST (Rochester, MN, where that CO flight was recently stuck) leaving in less than an hour, which went to MSP soon after. Got my friend onto his own flight home to FWA, and I hopped on the flight to RST, no problems. Rather than 9:00, I was home at 6:00.