I think it's extremely difficult, from what I've heard. I had a friend who had a 4.0 GPA, 2400 SAT, all the necessary EC's/leadership positions, had published research at Princeton, and didn't even get called back for the interview.
Besides that, the requirements that they impose upon you for your undergraduate studies are ridiculous. A 3.8 GPA and 36 MCAT?! If you have those stats at WashU, you can get into pretty much any med school you want anyway.
Personally, I wouldn't waste my time on it, as it's EXTREMELY difficult to get accepted to, and you will be constantly stressing over grades for your entire undergrad career.
Just my two cents, feel free to ring in/correct me, Zenith.