You can buy a prepaid sim at most AT&T or T-Mobile stores, T-Mobile does not offer data on their prepaid service, but you can get it on AT&T ($4.99 for 1MB or $19.99 for 100MB).
And yes, the US Prepaid services are absolutely pathetic compared to what you are used to in the UK.
Remember, AT&T operates primairily on the GSM850 band, so you will need a quadband phone. T-Mobile is mainly on GSM1900.