I'd also imagine that its somehow ordered by AOI, but I wonder how strictly. Unless you heard from someone in the department, I think you can only speculate...It may be, for example, that have 4 or 5 waitlists grouped by specific areas; or they may have just 2 very broad lists. As far as my own interests go, I'm primarily into history (Kant and German, as well as 20th century), but also hope to work on philosophy of perception, and aesthetics/method. What are you all interested in?
In any case, I was surprised by the positive tone of the email—as SteveZissou also noted. I'd wondered if they were that positive to all on the WL, because if they were its hard to imagine that everyone on the WL has "a good chance to be offered admission," haha.
Are any of you planning to visit the department in April? I'll definitely be visiting, and I'm excited to meet others who've been accepted/WL'd.
Good luck!