An aluminum backing would be nice. For one, they would eliminate the chances of breaking the glass (by dropping the phone) by 1/2. lol
In all seriousness, an aluminum back and an antenna would both help with any antenna issues. I wouldn't even care if it had one of the old pull out antennas for a classy look. I mean, if it loses the 'grip of death' and obtains a decent antenna with good signal, that would make a huge difference to the iPhone. After all it is an i-"PHONE".
An aluminum back would be good if they are going for a consistent look throughout their products. Almost every mac product is aluminum (besides the macbook) If the iPhone 5 is the same thickness as the iPad 2, and has similar buttons, it will then literally be like the iPads little brother. Be quite cool! But all I care about at the moment though is iOS 5 notifications and a redesign of the UI.
Come on! Aluminum iPhone! 8mp camera! 720p face-time! Antenna! 16:12 display! 64gb! Dual A5! and iOS 5 NOTIFICATIONS!
and who knows, even smart covers for iPhone 5?
It could happen!