The Detroit area has has its share of steampunk conventions, sadly they don't often last long. ;(
This one is brand new as far as I can tell. July 22-24. It's in the Detroit suburbs, in a city called Livonia. If anyone's not from the area, the place where the hotel is located is very clean, middle class, and low crime (You would be surprised to hear the number of people who hear "Detroit Area" and think they're entering Max Max or something). I have not been to a con at this hotel, in fact I've never seen a con at this hotel and I know ALL the normal con hotels in the area.
http://www.motorcitysteamcon.com/