No, you can participate (well, post answers/questions; no rep is gained) as an unregistered user without giving us an e-mail address at all (or even giving us a bogus one, we don't check).

If you do provide one, it lets you recover your cookie based account is all.

The "account" is as throw away as you want it to be, basically.

Sorry I was not clear and it is not terribly important. I've just wondered what it actually means to be "registered." If I have recovered my cookie-based account it means the account has been verified by e-mail. I'm just wondering isn't that a form of registration without having to have used OpenID?

Its mostly a distinction between "can login with one click (Registered with OpenId/Facebook)" and "can login with a crazy convoluted path (provide email [again], check email for link, etc.) (Unregistered)".

You can get the lion's share of a Stack Exchange's utility without registering, and this is quite intentional.

I just tried a test post and it won't let me proceed without putting an email address in.

Ah, my bad; been a long time since I've had to do anything as an unregistered user, mis-remembered.

Looking at the code, we do require something that looks like an e-mail, for recovery purposes (I assume, that code predates me by a bit). It is not, however, verified.

It appears to do a basic regex to validate the e-mail address but does not require a confirmation e-mail to be sent to post comments.