Congratulations to everyone on their acceptances and also to everyone still in the running! I can confirm there is a LOT of movement on the wait list. I believe about half my class was accepted off of the wait list, many during April and June, and especially May.
The best thing you can do if you are still waiting is send an update letter about something that will impress them (new grades, project, leadership, clinical, etc). I think it is also extremely beneficial to send a letter of intent if Wash U is your top choice. I sent 3 letters (1 update and 2 letters of intent, in March, April, and the very end of May, respectively). My update letter was substantially lengthy, about 1 page long about my current research and plans to publish (I actually wasn't even doing anything that new) . Even though nothing very new is going on, you should still update them with whatever you are doing (but with an impressive spin and how you are excited about pursuing it at Wash U)! Also include any good grades if you can, they really like new (good) grades.
My 2 letters of intent were a little less than a page and had a couple sentences of update and then mostly about how much I was dying to go to Wash U. I also made it very clear I would definitely matriculate if offered acceptance. And my last sentence was always a big thank you for considering me. I debated a while if I should send that 2nd letter of intent (Do I seem too desperate? Will they get annoyed at me for sending so many letters?) but it was definitely the right call to send it and I realize now how silly I was for thinking that they would get annoyed at a 2nd LOI saying how much I wanted to go to that school. I got accepted a week or so after that 2nd LOI so I know it definitely helped me! I think it is also good to send them a letter especially later on in the process (May, June) because I think they are getting less letters at that point and it shows you are patiently waiting and still very interested/fixed on Wash U (especially since most people have given up or are settling on a school at that point).
Some schools I applied gave me the impression that they do not like receiving letters or they had a maximum number of letters you could send. But I think Wash U is NOT one of them. As long as you keep them about a month apart at this point, I would say it is very beneficial and not harmful at all. I sent all my letters directly to the dean (Dean Dodson at the time, but now Dean Ratts), my interviewer (Dean Chung), and also CCed the admissions office. The dean always replied very promptly with a nice comment and said it was added to my file.
Again, don't lose hope! Half my class got in off of the wait list! There's so much movement, especially in May! You have worked so hard to get to this point (studying for all those pre med classes, MCAT, the application, interview) and you have gotten this far. It's now time to just give one last push in trying to impress them and tipping the odds in your favor! Sincere good luck to everyone! Almost there!