The CTCP website command and reply has the following command format and functions.
/ctcp TheNick website
will return client website referrer  information about TheNick.
SYSTEM TheNick CTCP: WEBSITE http://www.mibbit.com SYSTEM aWidget CTCP: WEBSITE http://docs.google.com/some/?widget=link SYSTEM AnotherNick CTCP: WEBSITE (blank)
and other variations, depending on the client.
If TheNick is using a Mibbit Client, the response is always sent. If the user has blocked their browser from sending the referrer, then the response will be shown with a blank field.
At the time of this writing, the Mibbit and qwebirc clients are the only irc clients that respond to this command.
It needs to be said, that some clients allow this information to be sent as lie, but generally they would not respond without some user interaction.