mmz6 and others,
All that the letter says is what was posted earlier: Everyone will hear something by April 15, and that you may be contacted earlier if specific decisions on your individual application are made.
I think that I may have gotten this letter a few days prior to the interview now that I think about it.
I also heard that some people hear back within two weeks and others may take months. I think this is dependant on how bad Wash U wants you (or doesn't want you). It may also have something to do with the timing of your interview and the ADCOM meetings, but who knows.
I have heard that on November 15, the Wash U ADCOM offers acceptance to 12 stellar applicants per region (4 regions) for a total of 48 initial acceptance offers. I do not have any idea how other offers are made or given out ( I think they also gave out some offers around Dec 10, but I'm not for sure). I'm sure there is someone out there that can offer more insight into this, but until then we will just have to wait and see.
Good Luck!
Bruce