A similar situation happened to me as the OP, and I was VERY uncomfortable because it was my very first interview and this conversation took place a few hours after the interview was over. I told them that it was my first interview and it was too early for me to tell, but the program has a lot of the characteristics I am looking for, and I would be in touch in December/January. But, yea -- I was explicitly asked where they sat on my rank list... the explanation was that if I wasn't interested, then they could offer the spot to another applicant (who might go somewhere else in the meantime).
It's definitely interesting. I also think it's an interesting dynamic to be put in the position where you may have to choose between, say, your #2 or 3 program (D.O.) who is offering you a spot in NOVEMBER versus waiting to see if you'll even get any allopathic interviews (or more D.O. interviews), going on those, and risk losing that D.O. spot because you weren't "eager" enough, or in my case, just plain ole' wasn't sure yet (again, it was my FIRST interview!!). I can't help but wonder if it is somehow playing into the whole keeping D.O.s in the D.O. match - give you the security of a spot early versus going into the unknown of the M.D. match... or maybe that's just my love of conspiracy theories.
To the OP, I say make a tentative rank list. If that program is in your top three, tell them all the things that you like about the program and that you will be ranking them "highly". If it's not, and you can reasonably get into one of your top 3, then say you'll be ranking them but you're still figuring everything out. If you are not doing the D.O. match at all anymore, if you wanna be nice, you can call and let them know that so they can put another candidate at ease.
To the other posters, yes this is TOTAL violation of the match agreement, despite the fact that the D.O. match is "different," the rules are technically still the same. I was warned about this two years ago from friends but had totally forgotten about it until my cell phone rang on the way home from the interview. Sheesh.
-K