You can repeat this test as follows:
1. Put your phone into "field test" mode by dialing *3001#12345#* (curiously discontinued with iPhone OS4).
2. Note the signal strength when phone is sitting on a table.
3. Note the signal strength when holding it normally or just touching the metal edge - it's way lower.
4. Pinch the phone so that you are not touching the metal edge. Note that the strength returns to the level it was at while on the table.
I have personally not been bothered by this limitation with the iPhone 3G in normal use. When signal strength is really poor, I avoid touching the metal edge, but aside from that, it's business as usual.
I would be curious to know if the iPhone 4 is any worse than the iPhone 3G/3GS. Has anyone seen a comparison?