But as slg mentioned below, it's trivial to check if a username/email is registered through the forgot password page. So some snooping significant other can still find out if they really want to.
The more secure systems I've been to don't acknowledge whether or not the account exists. I've actually seen a message that said something like
"If an account exists for that email address, it will receive an email shortly."
As I said (or meant to say) earlier, how anal you are has a little bit of luxury dependent on the nature of the site. If it's a social network, it's probably fine to show that usernames exist (unless their profiles are specifically flagged to be private), as part of the site shows that their usernames exist already. I could easily harvest usernames on HN here without ever having to test against the password form.
If your site is of a more personal, sensitive, embarrassing or whatever other situation you can concoct, then of course you'd want to adjust your username/password strategies accordingly.
