RPGNow have emailed everyone that was affected (ie: everyone who stored their CC info on their server). If you did store your info there but didn't receive an email, most likely your spam filters stopped it. If you didn't store your CC info on the site but your CC info was on the google cache page, please let us know (as that could indicate a more serious problem).
Last I heard RPGNow weren't going to email *all* customers (since they've already emailed everyone was was affected), but that could change.
(I agree with you that they should have a note on the front page - but I can see why they wouldn't want to. If you were visiting a merchant for the first time, would you trust them enough to buy something if they had a note on the front page that they had been hacked in the past?)
Cheers,
Jason