Acceptability letter is a fancy name for a post-interview hold. It means you didn't screw up your interview and are still being considered for acceptance while they interview other people. BU typically sends this letter out within a month after your interview. I have not yet heard of anyone who has not gotten it post-interview from BU, but I bet they have rejected some people. (just not here on SDN) <img border="0" alt="[Laughy]" title="" src="graemlins/laughy.gif" />
I don't think other schools use the term, and iof so, not many. Other schools just don't tell you anything. It's also NOT a wait list, just a hold status.
p.s. This was my 2,000th post! I didn't even realize when I was posting. <img border="0" title="" alt="[Eek!]" src="eek.gif" />