> Musk declined to say what 'MCT' stands for, and declined to answer further questions on the project.

There's some speculation [1] that MCT stands for Mars Crew Transporter (or Mars Cargo Transporter).

(The comments on [1] are very interesting, speculating on Musk's aims for a rocket that currently doesn't have that much value to private industry)

[1] http://nextbigfuture.com/2012/10/spacex-developing-larger-en...