By default, no it's not enforced, and that's
because Wikimedia is
about open editing, etc. You'll have to hack at the code to enforce
email addresses.
I understand the open editing thing. But I don't want to have people
register with bogus accounts. As long as their email address exist I
don't mind what they type.
I don't see how that impacts the openess of a Wiki in general.
Mischa