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.
2/26 - Goatwhore
3/3-5 - Berserker Fest III
3/18 - Witchaven
3/25 - Archgoat