Best PrePaid/Pay As You Go Network for a Brit road-tripping across the US

Hi guys,

I'm going on a three week roadtrip from next week and I'd quite like some advice on what the best PrePaid/Pay As You Go Network and tariff there is. I've got a iPhone 4 although it is locked to o2 in the UK so I'm not too sure if this will be a problem...

The route we'll be taking is from New York to Boston, then down to Baltimore, onto Cleveland, Detroit, Chicago and then South Dakota. Being completely unaware of what networks are good or bad and what coverage is like the the US, any help would be great. I was tempted to go with AT&T although I've heard lots of bad things about their network in New York, where I'll be staying for a week, so am open to other networks.

Look forward to hearing any thoughts or suggestions!