The sensitive parts of the antennas are the feedpoints. These are the points where the board makes connection to the antennas, usually via a coaxial cable or connector. For the cell signal, this point is at the lower left of the phone. For WiFi, this point is the top, near the headphone jack. It is typically easier to notice the effect of bridging the top gap by running one of the Speedtest apps while using WiFi.
The WiFi effect is noticed less often because people generally use WiFi in areas of relatively strong signal. If 3G signals were generally stronger (i.e., coverage was actually as good as what is claimed by the networks), then Antennagate #1 would have been much less of an issue. I live in Colorado and I returned my iPhone 4 because of the antenna problem (this was before there were so many case options). But, I was in DC recently and noticed (with the dBm meter enabled on my iPhone 3G) that coverage there is much better than out here.
These variations in coverage are, in my opinion, why this is such a heated topic: some people really do not experience reception problems. I had two friends out here that just purchased iP4s, and unfortunately they're experiencing all the same issues I had. The problem now is that they have to wait 4-6 weeks before they can get their free case. 4-6 weeks is anywhere between 0 and 14 days past the 30 day no obligation return policy. Pretty lame.