Apple has replaced the shattered iPhones 4 of three readers so far. One of them is Jared Franklin, the first guy reported to accidentally break his iPhone 4. "FYI - I updated my post, as Apple replaced the phone for free without any hassle," he told me in an email.
I doubt this is going to be a general policy because of the inherent fragility of the new iPhone, however. After all, it's not their policy with any other of the iPhones. Or maybe they would, to avoid any potential lawsuits. For now, however, these cases are just anecdotes.