Sonic,
Your success sounds great! I'm sure you will get multiple MSTP acceptances, your application must be very strong.
I've interviewed already at Yale, Pittsburgh, Hopkins, Wash. Univ., and Univ. of Virginia. I have MD/PhD interviews scheduled at Ohio State and Michigan, but will probably withdraw my application at both of those schools. I just got an interview invite from Univ. of Illinois Urbana-Champaign MD/PhD (Medical Scholars Program). But I'm probably going to drop my application there too. They are a non MSTP program and they dont give you as much financial support as the other programs.
WUSTL has a committee interview; thats probably the most important interview of the trip. There will be about 15 committee persons in there. Only 3 or 4 people will ask questions, the rest will just take notes on your responses and note their general impressions about you. The committee interview is only 20 minutes long, so it will pass by quickly. They will ask about your research projects mostly, but they might also ask you about why you want to do MD/PhD and some outside interests.
You will meet with various faculty as well, I believe that they provide some feedback on you back to the committee. If I remember correctly, 1 or 2 of these faculty interviews will be with people on the MSTP admissions committee. These probably carry more weight than the other faculty interviews. The last interview is also with an MSTP admissions committee person. This interview is sort of a wrap up talk, where you can express things that you didnt get a chance to talk about in the committee interview. Theres also a heavy dose of recruiting going on with that interview; its more informal than the others.
After your interview, the MSTP committee will meet the next week to decide whether to admit you or not. Brian or Andrew, the MSTP administrators, will call or email you exactly 1 week (unless theres a holiday or something) after the interview to let you know what the committee decided.
The MD admissions committee meets separately from the MSTP and less frequently. They will decide separately whether to admit you to the medical school. Usually this is just a formality for those applicants who the MSTP selects. I think there has only been a couple of people who the MSTP selected who were rejected by the med school. I get the impression that the MSTP committee generally gets its way, and has significant pull to get somebody admitted to the med school. So the limiting factor is the MSTP admission decision.
The interview weekend was alot of fun for me, the administrators did a great job of giving us stuff to do when we werent interviewing. Brian and Andrew are EXTREMELY cool guys, they are very loose and informal with the applicants. They will take you around St. Louis and set up dinners with current MSTP students. They also put you up in a very plush hotel, the Chase Park Plaza. Its probably the fanciest hotel I've ever stayed at.
OK, now I sound like a WUSTL recruiter, but I think you get the general impression. Great school, great location (definitely one of the better areas of St Louis), and great people. They have money flowing all over the place up there. I'm sure you will like it. I think the only thing that some people dont like is that its NOT Chicago, LA, New York, or Boston. Some people are averse living in St. Louis and want to be in a big east/west coast city. But thats really the only weakness that I perceived at WUSTL, and for me its not really a weakness at all.
Good luck with your interviews, I'm sure you will have lots of choices of where to attend.