Do any of the US-based pay as you go plans offer unlimited data? I didn't think that they did. I have an ATT plan a couple years ago that was month-to-month and got the unlimited data as part of it, but that was a monthly fixed rate. I did pay a bit more per month to go that route but if you're going to be pulling a lot of data that still might be the best approach. I think I ended up ~$85-90/month for the data plan and the lowest number of minutes I could get, including all the taxes and fees. That didn't include SMS and I have no idea what packages are available to make that work affordably.

I also do not believe that there is any plan from any carrier that offers free to any mobile phone regardless of carrier. Alltel is the closest with their MyCircle bit, but even that has a limited number of participants. Otherwise you're generally limited to folks on the same carrier.