I had the same question in my post here (I think): How to display custom filled map as background with other data on top (http://community.tableau.com/thread/122118)
The solution was provided by Tableau Tech Support.
1. Union the Polygon data with the Town Details data
2. Create a map with dual axis on the Latitude (one being a filled map of the polygons, and the other a "Automatic" showing the towns as circles). You then have to adjust the marks individually to format them. Attached is an example.