Anandtech has looked at this stuff in incredible detail.

The fact of the matter is that either the most sensitive region of the antenna should have an insulative coating, or everyone should use a case. For a company that uses style heavily as a selling point, the latter isn’t an option. And the former would require an unprecedented admission of fault on Apple’s part.

Apple’s dropped a bollock here. But – and it’s an enormous “but”:

reception is massively better on the iPhone 4 [compared to the 3GS] in actual use.

If you’re in a place with patchy reception – some people know it as “Scotland” – then the way you hold the phone makes a difference to the number of bars you see and the speed of your data connection, but the iPhone 4 gets a signal and can download data in places the 3GS can’t.