Not necessarily. I see this as incentive for developers to begin adjusting for 64 bit today so everything will be ready in the future. Apple did a great job transitioning the Mac seamlessly from 32 to 64 bit. Microsoft didn't do it so smoothly with Windows.
On one hand, mstone is complaining because Mail downloads every message. And on the other hand he is complaining because he thinks iOS Mail does not. Of course, with IMAP they both stay in sync with the server, as they should.