http://www.disboards.com/threads/foreigners-did-you-know.3441474/
See the above thread. Apparently Disney Policy is that a Passport will work, OR a colour photocopy of the passport AND another form of photo ID (driving license etc)
I'm planning on taking a few copies of my passport with me, and my provisional license.
A provisional licence would cost you a lot less to obtain just as ID than a new passport in the case of it getting lost (also you wouldn't spend days trying to get hold of a replacement just to be able to get home). If you were worried about losing your passport that is.
Elsewhere, outside of Disney, you will need your full passport, I doubt anywhere would accept a copy.