Thoughts on the new models maybe sporting the darker "gun metal" aluminum we've seen in the leaked iPhone 5 "unibody" casing? This darker coloring also seems to echo the navigation bar at Apple.com, which they changed sometime last year or so?
I'm not counting on it, though it could look cool. The problem with darkening aluminum like that is that it ends to show fingerprints and wear rather quickly. Structurally it's fine, but it looks ugly after some handling.
I also think it's unlikely because I don't think those iPhone backs are aluminum. We already know that aluminum isn't good for wireless reception, hence the plastic apple logos that allow wifi transmission on Macs & iPads and the glass covered polymer backs on the iPhone 4 & 4S. It's doubtful that the whole back is composed of stainless steel (though it's a possibility).
That may be the infamous Liquid Metal that we're seeing in those backs. What's cool about that material is that it can be made to suit any specifications. If the user wants it to look like aluminum, they can. If they want to change the color, they can. It can be finished in various ways as well. Frankly, if done right, Liquid Metal can be made to replace standard metal parts and look just like the parts it has replaced. Unless it's stated, most people wouldn't even realize it.
It can also be made to increase different performance factors in metals, for example, it can be made to better conduct wireless signals--and has already been used as an antenna.
Apple could have perfected fabrication for components of that size at this point. Something the size of a MacBook Pro? Probably not.