Sounds great Eliot! I will post a copy to my GitHub account and notify when its ready for review.
Re: some background on this feature. I believe it was Greg Studer that I spoke w/ on #mongodb about polygonal indexing and search. There was no support for polygonal clipping (aka query failed unless a point fell w/in the polygon search area; see: http://twitpic.com/67457c ) This is a much needed feature for spatial search beyond simple point data. Greg mentioned there wouldn't be much work in this area because of the interest to migrate to an RTree approach (ala. PostGIS, Oracle Spatial, etc.) sometime in v.2.1. Is that not the case?
At any rate, the implementation is primarily an extension of the following publications (w/ some original contribution and distributed modifications):
N. Beckmann, H.-P. Kriegel, R. Schneider, B. Seeger: The R*-Tree: An Efficient and Robust Access Method for Points and Rectangles. SIGMOD Conference 1990: 322-331.
I. Kamel, C. Faloutsos: Hilbert R-Tree: An Improved R-Tree Using Fractals. VLDB '94 Proceedings of the 20th International Conference on Very Large Data Bases 1994: 500-509.
I will keep this issue posted w/ updates.