Update:
My own testing seems to indicate that a Pages document created on the Mac saves versions that you can revert to but only on the Mac (can't find an iOS interface for Versions). However, if I create a document on Pages for iOS (forcing an upload to iCloud after each edit) I can't revert to previous versions made on iOS, even on the Mac. One time, when I tried to do this, I saw a message saying the file is stored on a volume which doesn't support permanent version storage. However I have been unable to reproduce this message.
As I understand it, we only have Versions on the Mac because we also have Autosave (on 10.8, ML). On iOS all saving (to the cloud) is done manually (the equivalent of the old command-s on Snow Leopard) thus eliminating the need for iOS versions.
I would be interested to hear perspectives from people who know more about how this stuff works.