I know that some in the area are CDPD (data on cellular), along with one county that was testing data on MPSCS. In other areas of the state, the unit of government will have their own freq(s) dedicated for this purpose. This is up to individual agencies.
We use Verizon air cards and some departments use sprint air cards. With the advent of 4g wireless around the corner it will make things even nicer. Talon is a new system....not new but a lot of agencies are switching to it. Oakland county agencies and some Macomb are on Clemis. But a lot of agencies can not afford clemis no more.
Many agencies have a dedicated frequency pair (usually 800 Mhz band) for MDT's to connect back to CAD. A lot of agencies, as MichiganPig mentioned, use wireless air cards. There was talk about having MPSCS support MDT's, not sure if that has come to fruition or is in experimental stages.
(Also the term "MDT" is falling from favor & being replaced by "MDC" - Mobile Data Computer; since the "dumb terminals" are being replaced by actual computers with more capability.)