[R-sig-Geo] Identify specific pixel neighbours

[Paulo Flores Ribeiro]

I would like to create a raster from an existing one, where each pixel 
in the new raster is set to 1 if the corresponding pixel in the existing 
map is in the direct neighbourhood (8 neighbouring pixels) of at least 1 
pixel with a particular value, or 0 if otherwise (e.g., create a raster 
that identifies all "agricultural" pixels that are neighbours of "urban" 
pixels). How to do this in the most computationally efficient way?
Thanks in advance, PauloFR