I'm guessing Comerica Park. A couple Warped Tours were there. Or how about Hart Plaza where the Detroit Electronic Music Fest and the Downtown Hoedown are?
Incidentally, I just landed back home in the dirty D for the holidays.
8/25 - BAT / ChainBreaker ?
8/30 - Morbid Saint / Vektor
9/13 - Sete Star Sept ?
10/3 - Prosanctus Inferi / Horrendous