[R-sig-Geo] kernels, contours, points?

[Jeebz]

Hi

I'm new to the R spatial packages, but I have a spatial-oriented task  
I would like to accomplish.

I have a 2d grid of points and each point has a class (13 classes  
total in the dataset).  The grid is small-ish, only 256x256.  I have  
run a 2d kernel density estimation (using kde2d) and can create a  
contour plot easily (per class).

What I would like to do is use a contour line to extract all the  
points from the original grid that are bounded by that contour.   
Ultimately, I wish to calculate a sort of "purity" measure, so given a  
particular contour (e.g. a contour level that encloses 95% of the  
classes points), how many points from other classes fall in with the  
classes that were used to define the contour.

I know there has to be a way to do this with R... right?  A colleague  
of mine showed my how it could be done using ArcGIS and "Hawkes  
tools", by calculating percent volume contours, but I don't know where  
to begin in R.

Anyone have any thoughts or examples on this?

Much thanks in advance,

bct