i got an email acceptance two days ago.
the way i believe they do it is the following:
everyone is preranked on a number scale going into interview day based on credentials, gre, etc. the interview then is incorporated into that scale (though based on the laid-back nature of the int, i think it was more to weed out the crazies or people who couldnt hold a conversation...so i dont think the interview changed the standing very much imo).
After this, i believe thay make a ranked list of every interviewed applicant based on their respective score. a first wave of acceptances is sent out (the people ranked 1-some number), then a second wave is sent out a bit later so that they can better gauge the number of acceptances and not send too many out. i would imagine the next wave will happen towards the end of next week.
good luck...hope this was helpful