The problem is that yahoo can custom-generate its links to DoubleClick so they encode your email address (dunno if they do do this, but I'm sure some sites and ad parters do). Therefore identifiying information is sent independent of the cookie.
Which is why one should have separate accounts created for anonymous use, and do everything (including setup of those accounts) from an anonymized connection.
Once you've touched your anonymous account from a session involving anything that *isn't* anonymous, it's game over.
~Mike.
