Mapping Center Answer:

One solution is to convert the polygon features to line features and then symbolize the lines.  We wrote this up for the Oregon Earthquakes map:

• Oregon Earthquakes: Creating boundary line features from polygons

If your map has a solid color background, here is a solution that was posted on this thread (http://forums.esri.com/Thread.asp?c=93&f=1730&t=233821):

"Another way to handle symbology on the redundant (duplicated) polygon edges where polygons meet is to create a two layer-linesymbol with a solid bottom layer symbolised as the background color of the map. On top of that one, you define the desired pattern for your boundaries. This approach will mask out the first edge when the second edge draws on top!" (Thanks to Bengt Lundstrom for this idea!)

To make the resulting dashed lines look even better in either case, use convert them to cartographic representations and set the properties for the dashed lines so that they stop and start the same.