Dude, I interviewed at four MD schools and one DO school, and got into four of them (I start at Cincy in four weeks!). Here's the deal: I'd guess that about 99 out of 100 applicants to DO schools also applied to MD schools. The DO schools know this, they expect this, and they don't care. Yes, you WILL be asked why you are interested in osteopathic medicine. And be prepared with an answer other than "I didn't get into any MD schools." I think you should be totaly honest. Tell them you were unaware of the existance of DO schools, and that you have come to understand their philosophy, and you like what you see. I suggest doing some research into osteopathic medicine, because they will see through your BS if you have no idea what a DO is. Also, they are fond of questions like: "What would you do if a Patient said to you 'I don't want to be seen by a DO, bring me an MD!'"
As far as MD schools go, none of them said a thing about my applying to a DO school. I am almost positive that they have no way of finding out that fact. If you are asked in an interview, BE HONEST! The worst thing you could possibly do is get caught in a lie.
Good Luck,