One of the things that really frustrated me about the KNN GIST distance box box centroid operators that came in PostgreSQL 9.1 and PostGIS 2.0 was the fact that one of the elements
needed to be constant to take advantage of the index. In PostGIS speak, this meant you couldn't put it in the FROM clause and could only enjoy it in one of two ways.
PostGIS 2.1.0 beta2 is out. Details on what's new in it are in official news release: http://postgis.net/2013/05/11/postgis-2-1-0beta2.
This is the first version of PostGIS to work with PostgreSQL 9.3, so if you are planning to experiment with PostgreSQL 9.3 coming out soon, use this one. Also check out the documentation in new ePUB offering format if you have an ereader and let us know how it looks. It seems to vary alot depending on what ePub reader used.
For windows users, we've got binary builds available compiled against PostgreSQL 9.3beta1 (and also available for 9.2 9x32,64) and 9.0,9.1 (x64). Details on windows PostGIS downloads page: http://postgis.net/windows_downloads. It does not yet have the new Advanced 3D offering (provided by SFCGAL https://github.com/Oslandia/SFCGAL), but we hope to have that compiled and packaged with the binaries before release time.