I've just tagged a "yt-1.6.1" release. This is primarily a very minor bug fix release. If you have an installation of yt-1.6 from the installation script, you should be able to "svn up" and "python2.6 setup.py develop" in the src/yt-1.6 directory to update your version.
The new command "yt instinfo" can provide information about the currently installed yt version, and in some cases may be able to automatically update your source with the "-u" option. This should allow for self-healing installations, as well as more rapid turnaround of bug fixes.
This release includes the following items:
* Critical fixes to ParticleIO * Halo mass function fixes for comoving coordinates * Fixes to halo finding * Fixes to the installation script
* "yt instinfo" command to report current installation information as well as auto-update some types of installations * Optimizations to the volume renderer (2x-26x reported speedups)