Does the iPhone operate on 3g only?

I'm contemplating getting the iPhone on a no contract network. I was reading a review online that said it operates on 3g only. Is that with all iPhone models on any carrier network or do certain carriers allow 4g usage for the iPhone?

As far as i know its only 3g

3G only

The new model of iPhone, due to be announced this Sept. 12, is rumored to be 4G capable.

I know through AT&T and Verizon that they offer 4G on the iPhone 4 and iPhone 4S. I also know that Sprint only offers 3g. I'm not 100% sure about any 3rd party carrier.