I'm just writing this HowTo for other people who occasionally pitch up in the USA and who sincerely wish that their GSM smartphone would just basically behave like it did at home - i.e by allowing them to use the internet as much as they want without incurring a bill the size of the national debt.
The last time I was in the US I invested a half day and found the following is the best solution for GSM using travelellers in search of unlimited mobile broadband to use in their existing handsets, without subscribing to anything ongoing or having to get a new phone. This is the fruit of labours:
1.) Go somewhere you can buy a t-mobile phone or SIM card. I went to Best Buy, but it took a bit of hunting to find somewhere with T-Mobile stock, especially on a sunday.
2.) Get a T-mobile pay as you go sim card for what they currently call 'Monthly4G', the one at $50 per month, with 1500 minutes included. The total will be about $60-80 plus tax depending on how much they charge you for the SIM itself, and whether they can actually sell you one without a phone.
3.) Put your new SIM card in your phone, follow the various activation instructions that will be SMSed to you - if you're in a shop the staff can help you through this, as they did in my case.
4.) Once this was done, I got connected to the data network without actually having to change any phone settings at all. I was very pleased, and not inconsiderably surprised when it 'just worked'. I was able to tether, too, using my PC.
The SIM also came with 1500 minutes of calls within the US, so all the phoning I had to do with people there was all covered by my investment. The next time I go back all I'll have to do is add the $50 again, and I'll have up to another month of unlimited data, plus more calls than I could possibly use.
If you do stumble across this post whilst trying to solve this problem yourself, please leave a comment to let me know what the current state of the art is.
PS You've got to have an unlocked phone, should have mentioned this above. But I reckon if you're even reading this post, you know already.