Remember that iPhone 4 prototype that was left in the bar? It was reported that it had this plastic thing around it so it would look like an normal iPhone 3GS.
That was their field test and maybe they didn't even notice the issue because the phone was always hidden in that plastic-camouflage thing and nobody came to touch the metal underneath it.
Just my 2 cents...
Since they designed the break between the two antennas to stop conduction between them there is no chance that they didn't realize that a person holding the phone could make a connection with their body.