Well, I'm an accountant, but I don't do tax so this is just my thought. I think I remember from basic tax class that you have to subtract the value of anything you received in return. For instance, if you bought tickets to a charity dinner for $300 but the dinner was worth $50, you would use only $250 in your calculation for your charitable deduction.

So, you would have to subtract the "value" of the miles. Seems like a lot of trouble. Maybe just the good feeling of getting miles cheap while donating is enough? Of course, the IRS probably won't know about the miles...it will be up to each person.