Giraph was the early adopter of Hadoop YARN AM! Eli successfully wrote a Giraph AM based on Hadoop 2.0.x_alpha. However, in last few months, Yarn significantly overhauled its APIs and associated coding patterns. The new beta version is 2.1.x and I was told by Yarn-dev that current APIs will not change much.
In the above circumstances, we need to substantially overhaul Giraph AM as well to accommodate with the new Yarn API. Moreover, in newer YARN API, supporting kerberos security in AM becomes easier and more transparent.
The upcoming Girpah AM will not work with earlier alpha Hadoop versions such as 2.0.3. I'm not sure if anyone is using Giraph AM in production. However, the more prevalent way of Giraph processing (MR-based) should continue to work.