He's really an auxiliary bishop in name only these days. He "resigned" his adminstrative diocesan responsibilities some 15 years ago, and became the pastor of an inner-city parish. About the only episcopal duties he still performs are ceremonial ones and conferring Confirmation as his turn comes up in "rotation".
There's really no point to the Vatican accepting his resignation, since they'll probably feel obligated to appoint another auxiliary in his place. I'm sure they would much rather utilize their declining number of available bishop-candidates for vacant sees and growing dioceses with greater needs than Detroit. I'm pretty sure this is much the same reason that they have delayed accepting the resignations of Cardinal Maida and Bishop Mengeling in Lansing.
The "rust-belt" dioceses are not exactly a high priority for the Vatican. The historical precedent of the Detroit ordinary being named a Cardinal is likely to end with Maida in favor of the Archbishop of either Houston-Galveston or San Antonio.
I was wondering if we were going to retain a Cardinal. I guess things are on hold until more of the young men attending the seminary are ordained as they seem to be more conservative than their predecessors. A number of the newly ordained are being persecuted by their brother priests who believe the seminary is too conservative.