For the US, the car would need a larger engine and additional crash tests. I can't indicate how long this will take but it could be three years.

If we go by what the Tata Motors Chairman Ratan Tata said at Delhi Auto Expo 2010, the US needs to wait for three years at least to see the first Nano taking to the roads.
Mind you, at the Geneva Motor Show 2009, the car-maker had unveiled the Nano Europa that touted high fuel efficiency and less carbon emissions. However, the technical specifications about the American version are scarce. Still, what we can be sure about is it will be a modified and safer version.
Via: CleanTechnica