I was starting to have the same fear, because I faxed the reply in 4/14. A week later I saw something that said it should be sent by surface mail. Aaah! Then why put a fax number on the stupid form? (That would make for a great, implausible story to tell - yeah, um I could have gone to mit but I didn't send the form in the right way.)
Anyway, a newsy email came yesterday, with my info on how to set up email, get web certificates, etc. I just talked to a woman entering a different dept and she heard earlier this week. So apparently it's just a matter of when your dept. sends it out - I wouldn't worry. Maybe email your dept secretary if you're eager to get the web certificates.
It's funny, part of me expected more, um, fanfare when I accepted the offer. I spent so much time on the decision it seemed kind of anticlimactic to have them just say "ok, great, welcome to MIT." I don't know what else they could say. When I was entering undergrad my school sent me a big packet with the school song and stuff, but that would seem a little childish at this level. But still.