This is a simple geoprocessing tool that extracts adjacency information from a layer of polygons. The output is a polyline layer with one record for each border between each pair of neighboring polygons.

Download v. 0.1.

After downloading and installing, you can right-click on a new toolbox, choose "Add -> Tool", and select Boundary Generator under the "MWSW Geoprocessing Tools" group. You can also find MWSW Boundary Generator on the "Index" toolbox view.

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Project status (alpha):

• Generating boundaries works; initial spot checks are giving correct answers.
• Some of the Geoprocessing UI behavior isn't quite finished -- e.g., you must specify the output location yourself; the tool gives an erroneous validation error if you try to run it on a layer that isn't in your TOC. (Expect these to be fixed soon.)
• The tool hasn't really been tested in a model builder / python scripting framework yet, and needs some adjustments to work well there.

Project rationale:

• It is a good demonstration project for custom programming geoprocessing tools in C#, as well as simple computational geometry & spatial indexing techniques.
• It is robust and geometry based; the other extensions I've come across for doing the same thing simply searched for line segments with identical endpoints (and would thus fail with partial overlaps, T junctions, numeric precision issues, and the like) or used polygon-polygon intersection to come up with polygon adjacency lists (but not accurate border lines), as described in this old article.
• This functionality is available without an ArcInfo license. (If you do have ArcInfo, you can use the offical Polygon To Line tool. I would be curious to see the results of a side-by-side comparison from someone with a license...)

If you are still using ArcView 3.x, my (dated and not well-tested; beware!) Avenue script for doing the same thing is available at http://arcscripts.esri.com/details.asp?dbid=12786.