3G is “third generation” wireless data service. There is no 3G iPhone, because 3G wireless data is not broadly available and is a significant drain on the battery life of the phone. Pundits predicted that the iPhone would be a flop without 3G data service; then they predicted that the iPhone would slow in adoption without 3G; now they’re predicting that there will be a 3G iPhone any day now.
Steve Jobs, who is in a position to know, has said that 3G will not be in the iPhone until it stops causing problems for battery life.