Last year I sent in some MM winning tickets (not as many as I would have liked to send....but, I digress....). I never mail them certified anymore because I've never had an issue. So after a month or so I emailed Harrah's to check on the payment, see whether I should mail them copies, etc. Without any more info than my name and email address, they told me the amount of the check and the date it was sent.
Kinda scary, in one way....but I think this doesn't happen (at least not as quickly) if I hadn't provided my Harrah's card when I made the bets.