(Quoted from here)The mental ray team is inviting you, the experienced mental ray user, to contribute to the fine-tuning of the features by trying out, and hopefully stress-testing, a preliminary version of the mental ray 3.12 Standalone. This software will be licensed for a limited period of time, but otherwise has no restrictions. It will provide the full functionality of the final product.
http://www.nvidia-arc.com/products/ment ... -beta.html