GSM 1900 which I think makes it compatible with (most?) US networks.
You can't get "most" US networks, you can get the two main GSM networks with 1900 coverage, T-mobile US and Cingular. You should be ok in NYC with 1900. You cannot use Verizon or Sprint as these are CDMA networks, not GSM. Verizon seems to have better coverage than the GSM networks today, but Cingular are expanding very quickly nationwide.
What I am finding hard is getting hold of a really cheap sim card from a US carrier. All I want is a number I can use to give restaurants and such like for bookings. I won't be making many calls and if I do they will all be local. Can anyone suggest where I can get a cheap (or better yet, free) sim card?
The US GSM networks are not as advanced as Europe, and when last asking in 2003 there was nobody selling SIM cards alone. Note that USA prepay is similar to old-style in the UK where you had to pay "service charges" on a daily basis, or you were disconnected.
The only thing I've found via Google is -> http://www.cellulara...com/usppsc.html
If you risk it, let us know how you get on.