I've been experiencing the iPhone 4 in hand reception issue since my phone arrived on June 23rd. I've read some comments online from people who said they couldn't reproduce the symptoms in the Apple Store. Well today I happened to be in an Apple Store and I was able to reproduce the exact issue I've been experiencing with one of the display devices. I didn't have to do anything special to recreate the issue with the display device other than hold the phone exactly like how I used to hold my iPhone 3g. BTW, the video was shot with my iPhone 4.
http://www.youtube.com/watch?v=X3dMhyti02o
http://www.youtube.com/watch?v=X3dMhyti02o