I have traveled to Mexico with the iPhone on business a few times now. I get the international data global plan for 50Mb and disable all automatic checking (but keep Data Roaming on), and I also get the Mexico calling plan (not sure of its name but it makes the phone part cheaper). Then I enable Wifi and enable the phone asking me to connect to wifi signals.

The next step is to just be conscientious on the use. For example I open the laptop in the morning and get rid of all spam, etc, on the laptop so my inbox stays tidy throughout the rest of the day. I also use some server-side mail rules - procmail in my case - so the inbox doesn't get clogged with mailing list stuff unless I want to see it. Resetting the counter I haven't done but it's a good idea so you can keep tabs on your usage. Even with that I still get an overage of about $100 on top of it for a two week trip, which I arrange to expense with my company.

In Mexico there is a system called Prodigy Movil which is similar to T-Mobile's Wifi. Most airports have it, and a lot of restaurants and other public places do as well. If you have a friend with TelMex as their provider you can have them arrange for their monthly unlimited plan for $22 (or if they have TelMex broadband but no laptop, maybe they'll be nice enough to lend you their credentials because this service is free to their Broadband users).

My hope is that with the iPhone going international the phone companies will start taking a closer look at the data plans which are a complete rip-off as they stand today. If they end up with all you can eat plans throughout, maybe we will be able to pay for a month's worth and then not worry about it?