The "Ask Sandy" blog on BW has a lot of details about interview requests for HBS. I didn't look at it closely because I don't have an application in, but I do recall something like "if you don't have an interview by Christmas you are dead meat".
It was a lot different when I was applying for law school. Applications were all on paper and decisions were all sent by mail. Generally there were no interviews for law school. The rejections were pretty much sent out as soon as they rejected you (I think Yale must have decided to reject me after about 3 minutes I got the letter so fast) but acceptances were sent out close to the deadline.
Did you guys read about the incident at HBS a few years ago, when a bunch of students (like 500 if I recall correctly) got wind of some way to get a sneak peak at their decision by somehow hacking into the system? All those applications were rejected - so don't do that.