I doubt they would do this. The 3 million people that bought one already would be pissed if any modification was made, including a sprayed on coating.
From a business stand point (that is to say, a financial standpoint) Apple already made out like bandits in the first month. It would be way cheaper to keep selling the i4 as is and then launch the i5 (or whatever they will call it) next summer.
I hate to beat a dead horse here- I know a lot of people are having issues, but a lot of us are not. I would never settle for the antenna/proximity issues a lot of users are having. I would simply return the phone. There are other choices out there and if all you do is complain about the iPhone 4 than perhaps it is not the choice for you.