This is a very interesting project, it's filling an huge gap and soo many projects need to reimplement these structures.
I have one early question, have you considered pushing the quadtree and rtree classes into boost::intrusive, or designing them in the spirit of boost::intrusive? The audience who's in need of large spatial databases (the ones who doesn't need it are fine with linear scans) is likely to hunt for performance. IMO boost::intrusive has given a flexibility to c++ containers not available elsewhere, it'd be a clear winner if your quadtree and rtree can be implemented in generic terms.
Christian, I don't have experience with boost::intrusive, but I'll check it out to see if it fits with the spatial indexes. Thanks for your suggestion! -- federico