Due to recent issues that arose after we shipped the b12 release (the Firefox 18 Web Worker bug and the Bing Maps key expiration), we decided to put together a release outside our normal monthly cycle to get these issues addressed for a wider audience.
Blog post is here with a summary of other changes since b12.
We also rewrote the quick start guide to more clearly describe the contents of the release zip, and make it clear that the contents are ready to use as-is, with no build process needed, whether you’re using the standalone Cesium.js, the AMD modules, or the CesiumViewer application.