Shake to Shuffle may be fine on a tiny iPod Nano, but the iPhone is big, heavy and slippery. I have a feeling that it would lead to a lot of broken iPhones and Nintendo Wii-type accidents like black eyes, smashed windows and cracked kneecaps!
Its not coming and probably for the better part..... iPhone is Apple's top mobile device (something which Apple has put to question lately) and its supposed to be somewhat a royalty and I dont see Apple having people shake its most acclaimed product to death and not to mention the high risks in shaking such a curvy slippery heavy product!