Looks like Apple really has improved the sounds quality on calls. Both GSM and 3G.
From iPhone 3G review – Engadget:

One thing Apple was keen to talk up is the vastly improved call quality of the iPhone 3G. Those in the know understand that 3G call quality is often better than regular GSM — but it turns out Apple made a huge improvement on both sides. iPhone 3G calls made over 3G and GSM both sounded significantly better than calls made on the original iPhone. If you’re upgrading your device iPhone you may not necessarily notice it, but on a side by side it was pretty obvious.”