In the US...we only have the 3GS networks in 3 cities...St. Louis and two others I can't remember.
If you bought the 3GS for faster wireless bandwidth, you're SOL...the iphone 4 will be out long before the network infrastructure is updated in enough places to make it worthwhile. Granted, you may see faster loading times IRL right now--but that's because of the faster processing of the data and not the higher bandwidth.