I really like the aluminum back. I loved the look on my original iPhone (which is still the best back-casing I've had on any iPhone, and I've had them all), I love the look on my iPad, and I love the look on my 2011 MBP.
My only issue with the original iPhone was the black band at the bottom. If Apple could run the wireless through the Apple logo, and then maybe through the power button, or headphone jack, or dock connector (kind of like how they do that with the superdrive slot on the unibodies), then I'd be game.
While I think the iPhone 4 looks great, it's edges are too sharp, and I feel like it is more prone to breaking than any iPhone I've had before. I got my wife a 4th gen iPod Touch the other day, and man, if I could get that as an iPhone but with the non-glossy aluminum back and slightly more rounded corners then I'd be so happy! If they could bring the edges of the device in further so that the screen goes all the way to the edge, then that would be even better!