Aluminum, just because Apple can curve aluminum. Having the device entirely flat is nice visually, and it's nice for positioning the device, but for actually holding it and using it, a curved back is more comfortable.
I hope the new iPhone is a liquid metal design (it is RF-transparent, unlike aluminum). Although, the white front won't look quite as nice in that case.
I like the "premium" feel of the glass, but as mentioned above, a liquid metal back would be awesome. It would also be nice to not have to worry about the front AND the back cracking.
To be honest, I have dropped my iPhone MANY times, and of the 4 iPhone 4s that I have had this year, the back is the only part that ever took any damage. After daily use for about 6 months with no case, the back had little scratches all over it. The front was still completely flawless.
I think the glass on the back is actually weaker than the glass on the front.
I hate the glass back. I tried one of the metal back pieces for awhile on my iPhone 4 but didn't like it. I have "leather" sticker type thing on the back now. The glass has too many cons IMO; looks gross from finger prints, does not provide good grip, easily scratched and then looks like ****.
Give me a matte finish for science's sake. I hate shiny.
I vote glass and even more so now that I have the white phone. Finger prints on BOTH sides drove me a little nuts so I kept a Case Mate Barely There on my black phone the whole time. Now that I have the white one, I only use a bumper (until I got the BookBook)
I'm gonna say it'll have the same type of Aluminum that's on the iPad2. Very nice and elegant look and feel of an expensive high tech device. Also, the possibility of being anodized, but not 100% on that one either.
Would be cool if the apple on the back lit up like the mbp too. But that's just wishful thinking!
That would be cool, perhaps they could make it glow when the phone is in use or receiving a messgae or alert (for those who must have an LED notification). Of course battery tech would have to greatly improve or a smaller separate battery for it. Still wishful thinking.