It's less than one additional dropped call in every 100... For me, my 3Gs maybe dropped one or two calls in every hundred calls. . . Usually when going through a cell tower change in rush hour. I do know that I am getting better reception from my 4 than I did with the original, the 3, or the 3Gs I had before it.
So, if it was less than 1 per 100, then with an iPhone 4 - per Steve Jobs - it would now be less than 2 per 100. About twice as often. Still a small number, but for a magical antenna configuration, doubling the number of dropped calls doesn’t seem all that special...;)