Epi - what you did is fine and is not, for some reason, seen in the same light. Not sure why. I sent ONE letter of intent last year, to UCSF, once I was told I was on the top half of their waitlist. I had already signed an INTENT TO REGISTER paper with another school. When I got into UCSF, they told me I had to withdraw all my apps immediately and decline my acceptance from the other school. When I called the other schools where I was waitlisted to withdraw, they KNEW where I was holding an acceptance. So, the schools do talk (and it was after May 15).
As for the letter of intent to several schools issue, I brought up the "contract" issue as an example. I see letters of intent - when initiated by the applicant for purposes of garnering admission - as similar to an EDP application; as a result, there are implications if you say you will go to more than one school if accepted. I remember reading something in the Kaplan guide to admissions (not sure of the exact title) where it said that you have to be very careful with these types of letters because the committees do talk to each other and a school can "call" you on it and try to force you to attend. I believe AMCAS can get involved in this (Lilycat had questioned this), but am not sure. In short, it's not a wise idea.
As for the saying every school is your #1, I initially had ethical issues with this as well, but I was told that the schools usually expect you to say this to them. And, you should keep in mind that for many applicants, the school they get into IS their #1 choice (only option) because no one knows where they will have luck or be rejected (i.e., your options change throughout the process -- if you only had one top choice and were rejected, then your top choice will change. This can happen many times throughout the process, so if you truly like a school and would like to go there, then I think it's ok to tell the adcom their your #1).
Anyway, just my thoughts. Good luck to all!