Extracting polyline of border between differing polygons using R or GRASS?

I have a shapefile with polygons with an associated variable that can take different values. I would like to extract the borders between polygons of differing value (red and blue) as a polyline (dark black line).

I would prefer solutions in R but will accept other open source solutions.

Answer

Using GRASS it would be a simple two step process. After importing the polygon vector into an appropriate GRASS location, you would run:

v.dissolve  output=
 column=

then, to get just the boundaries:

v.extract 
 output= type=boundary