Day-to-day, the speed difference between the iPhone 4 and the iPhone 3GS isn't that different. But in more intensive apps and games? The iPhone 4 smokes the 3GS and is nearly as fast as the iPad.
Playing with an iPad, one of the first things you notice is its speed. Everything zips into place. Our side by side tests comparing it with the iPhone proved that (for the most part) its snappiness isn't just imagined. Updated.