It introduces plug-able JSON back-ends, like the plug-able XML back-ends that exist today. It ships with JSON back-ends for the existing YAML implementation and the JSON gem. In addition to that all of the plug-able back-ends (XML and JSON) support parsing from an IO as well as a string now.
technoweenie has written a Rails plugin adding yajl-ruby as a back-end at: http://github.com/technoweenie/yajl-rails/tree/master
I also have a fork that I'll keep up today with the latest API changes in yajl-ruby at: http://github.com/brianmario/yajl-rails