A couple of years ago on an extended trip through the USA, prepaid data was definitely an unknown quantity -- it may have changed now and someone else on the forum may have updated information. In the end I had to call on a good friend to actually take out a contract on our behalf and then take over the remainder after we left. Now THAT's a good friend!!!
However if your 3G card is multiband (i.e. has the usual 4 worldwide bands, which I think most do in SA) you will definitely be able to use it right across the States. Be warned though that contrary to what you might expect from 'the world's technology leader', coverage can be very patchy, and nonexistent away from urban centres and main highways. I often struggled to find a signal while travelling, sometimes for days at a time.
