Well considering I interview at DMU....... I guess I could say something.
First, what I believe is not what everyone else does, but I feel that more are like me then not.
I really don't care if your letter is from a DO or an MD or even someone else. I personally sent two letters, one from an MD and one from a RN, who was my boss.
What I care about more is:
1. You have medical experience and your letter is written from someone who observed you in that experience. A letter from a nurse about your work ethic, patient interaction, and desire you show at work on a daily basis is more meaningful to me then a letter from a doctor who you "shadowed" for an afternoon and did an interview with.
2. You know what a DO is, what a DO does, how they are different, what the DO philosophy is, and you have talked with a DO or representitive of a DO school.
3. The person who writes your letter can relate to me about you personally. I want to know your the person. I know you the student from your transcripts. I will get to know you the personality from your interview. I use the letter to get to know the you that isn't presented to me...when possible. Some letters just don't convey much. Only a bad letter will hurt you though. If your letter is pretty standard it doesn't work against you.
Finally, if you actually knew the DO that your followed and it wasn't just an afternoon and 3 hours of interaction, I would use that one. That way I know that you know what it means to be a DO.