I'm going solely on the basis of the libyaml docs here: http://pyyaml.org/wiki/LibYAML

I wasn't able to find a good "Changelog" of what the differences between 1.1 and 1.2 even are.  PyYAML, of course, claims to support 1.2, but uses libyaml under the hood, so I don't know if all of those differences are happening at a higher level.