New York sim advice for traveller from the UK

Hi - Hoping someone out there can give me some advice please. I am heading from the UK to New York tomorrow and was hoping to use my iPhone 4S while I am there. I called my carrier, O2, just now and they don't offer an international data package, and their rate for data is £6 per MB!! (probably roaming on 3G at&t which will be crap anyway in NY right?)

Can anyone recommend the best pre-paid sim for me to get when I arrive to work with my GSM iPhone 4S? I'm only going to be in New York for 48 hours (a work trip not a holiday) so I think 1GB should be plenty.

Thanks in advance!