I have never come across someone using one but it is valid. I would actually hate to see someone try to use one. I come across enough issues trying to use '+' in my gmail email address.
It's not just TLDs. Machine aliases are also perfectly valid in e-mail addresses, e.g. "root@localhost", "fred@finance" etc.
This might not be practical in a majority of applications (you're hardly going to sign up to 3rd party services using an alias to a machine on your local network) but if you're building a generic e-mail address validation library, it's an edge case you cannot ignore.
I have never come across someone using one but it is valid. I would actually hate to see someone try to use one. I come across enough issues trying to use '+' in my gmail email address.
http://en.wikipedia.org/wiki/Email_address#Valid_email_addre...