There's absolutely nothing to stop Apple from releasing a new version of the iPhone before June 2011, whether it's and iPhone 4S (revised hardware) or iPhone 5 (redesigned hardware), and if they did current iPhone 4 owners would not be entitled to an exchange (assuming they're outside their 30 day purchase window).
Other than the fact they're selling the current design as fast as they can make them, I agree with you. But I still can't see a truly compelling reason for them to change the design with the current circumstances.