The metal backplate is another feature we've seen in leaked photos and videos. This is entirely reasonable, and would not only allow for a unibody-type design if Apple chose it, but would eliminate the glass back of the iPhone 4 and iPhone 4S, which are prone to shattering when dropped. It would mean a lighter phone, extra durability--and fewer angry customers with broken phones.
Probability: We say it's happening. There have been enough photos and video leaks to orove it's true.
(Image credit: chrisbrownie91 on YouTube.)