badgergirl,
From what I gather, I don't think the decision is binding at all. Rather, the letter mentions that they need some kind of an idea of the student's interest so they can better organize their class. It makes sense to me -- after all, Pritzker is a very practical place. Otherwise, you are right in that May 15 is the final binding deadline to notify them one way or another.
Best of luck at your interview in January! I don't think I've been more impressed with the caliber of my fellow interviewees than at Pritzker. You guys will all bond (the group is usually 6-8) really well, and they'll give you a picture at the end of the day with your interview group. Remember, if you interview there, the odds of getting in are quite high!
If it's your top choice, communicate this with Sylvia or Dean Lombard (you'll interview with one of them for sure) -- it counts for a lot! I also recommend staying with a student host if you get the chance. They provided invaluable insight and even knew some of my interviewers the next day! If you need any more insight into interview day or Pritzker, feel free to PM me.
P. Parker