Two other things to note about prepaid in the US (two biggest GSM providers here are T-Mobile and Cingular):

1) If you get a tri-band phone, you'll be better off (for now) with T-Mobile prepaid. There is some overlap in spectrum with the gear they use in the US and in Europe, so reception shouldn't suffer. Cingular is a bit more dicey.

2) T-Mobile recently developed a glitch in their system that allows prepaid users to get free data access. Not necessarily a compelling reason to choose it since they can fix it at anytime, but you never know when it could possibly come in handy.

There is no question that you want to buy at least a triband phone if you visit the US, which the OP implied. A word of warning: In the US, triband phones often skip the essential 900mhz band replacing it with the 850mhz band. If you have to pick between the 850mhz band and the 900mhz band, go with 900mhz. 900mhz is used in many more countries than GSM 850mhz. There are only a handful of countries that use exclusively 850mhz GSM. Conversely, there are numerous countries that are 900mhz only. 850mhz gives you better coverage in the rural US, but you'll give up coverage in places like all of China (except for the Hong Kong SAR), most of India, and even more rural parts of the UK.

If you can afford it, get a phone which is at least a quadband. I have a sect-band phone (quadband + 2100mhz UMTs) and don't regret it. Quadband will get you coverage everywhere but Korea and Japan. 2100mhz will give you coverage in Japan and Korea plus everything else. 2100mhz will also give you high speed data in the UK. On TMobile's 3g prepaid plan, you've got unlimited PDA data for a £1 a day. It is soft capped at something like 20 megs a day, but I exceeded it last week and didn't here anything about it. Laptop tethering can only be determined by the quantity of data consumed on most phones.

Well, I guess I might as well say it now; T-Mobile is the one that uses 900Mhz whereas Cingular uses 850Mhz.