iPhone best buy: new iPhone 4G - wait for release or buy Apple iPhone 3GS?

My dad wants to get a new smartphone because his current phone has an annoying touch screen. The phone itself still works though. He’s looking for an Apple iPhone, so should he buy the Apple iPhone 3GS now or wait for the release of the Apple iPhone 4G. Do you guys have any idea for when the release is due, and what will be the advantages of the Apple iPhone 4G over the Apple iPhone 3GS?