Well, there are lots of ideas here. Donald suggests September for Yellowstone. I would suggest later in October, as the elk then are in rut and the colors are in their prime. I am already pretty tied up from the middle of September until the middle of October with meetings and guests from overseas. May gives the same visitor use level benefits as September and October, and probably a bit lower, but in my mind, that is not too much of an issue.
As for meetings during the day, that is probably an issue, as generally speaking when i am in the park for photography, my philosophy is that no daylight should be wasted. In May, for example, the wildlife is out all during the day, not just during the mornings and evenings. May also allows for more daylight.
All that said, any time in Yellowstone is better than no time. msmoto, sounds like flying is cheaper than driving for you. The appropriate airport is Bozeman, Montana. If people would like to rent a van, I can help with arrangements for that as well.
I will probably go down there in January for some photography as well.