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.
1/15 - Mayhem / Watain / Revenge ?
1/18 - Pharmakon
1/22 - Vattnet Viskar
2/8 - Aesop Rock ?