Actually I spoke with the graduate administrator the first day acceptances were sent out (via email) and she told me that the letters were being sent out that day and that the DGS would be giving 'heads-up emails/calls.' While this is rather ambiguous I got the distinct impression that letters were in fact going out in the regular mail. Also, if she did mean that email attachment letters were being sent out that day then why have so many people not received them? It is now 2-3 days later after the initial acceptances were emailed so it seems as though the snail mail route may be true.