I believe this issue came up when someone posted a way to break into the HBS status computer - Somewhere in all the legalize there a blurb that says they have the right to change offers, change status, etc. So even if you get an email, they could change it, not honor it etc. In the HBS example, those that had an admit found a nice DING a little later.
I think trying to "force" them to honor an admit email is not even reasonable. As I'm sure we all know, email is easily spoofed -- I would just send myself the admit email then respond I accept, and voila' -- be in every school I choose.
For that matter, yes, Scarlett Johnason, I do want a bigger unit, and I'll be glad to call you and arrange a date -- cause your email said you'd hook up with me!
Later -- RJ