They now have it at Baja Fresh around here. It still hasn't appeared in any Wendy's within a half hour of where I live.

Not really midwest, but I noticed two places so far that have Coke Zero in the fountain.

Chick-Fil-A, standalone at the Miracle Mile Shopping Plaza, in Monroeville, PA
Wendy's in Monroeville, PA

Wendy's in Kansas City has it at their fountain.

Outback is the latest big restaurant chain that just got Coke Zero

they might have it at the royals staduim in kanas