How big is that section? Because according to my experience, it would be the entire southern half. That is just crazy and isn't fair at all. Every time I go there I am completely cut off from the world and can't get directions to drive home.
I don't seem to have problems with service between Milwaukee and Madison, both of which are in the southern half.
Keep in mind, you can download maps from Google so that you can have mapping information if you are unable to connect to service.
We understand how important your service is, especially while traveling. We want to get to the bottom of this. As rcschnoor stated, there is coverage in the lower half of the state. I myself have traveled to Minnesota, Wisconsin, and Illinois and had no trouble crossing the border and using my data. We would love to find a solution for you. Can you provide you make and model phone? Have you attempted to restart the phone once you are in Wisconsin?