Seems Nokia tried to poke Apple about the issue publicly, only to have it backfire with people digging up manuals from Nokia phones advising users not to touch the bottom, and demos of some Nokia phones experiencing the same issue.

http://www.appleinsider.com/articles/10/...lled_on_it.html

So it's definitely not an issue with just the iPhone 4, as the above story shows videos from several phones, including the Nexus One. And as Tony and others have explained, yes, part of this is normal, though hopefully future phone designs will better account for their customers holding them.

For me, I generally don't cup the phone, I hold it with the left edge against my palm, and my finger tips holding the right edge. So for the previous iPhones, this never blocked any major portion of the antenna, but with the iPhone 4, it's connecting the two antennas together to cause my phone to drop signal strength quite a bit. I can't get exact DB numbers since it seems the field test app is no longer accessible on iOS 4. And so far, the Apple store reports being out of stock on the bumpers every time I check. I will continue to "hold differently" for now, to see what Apple's formal fix is.

*edit*, found this good video demonstrating it's not an issue with covering the antenna, it is indeed just "The Spot". http://vimeo.com/12864890