It might be a bit early for me to give away too many details, but... The atmospheres in the shots I pointed out were a mixture of Terragen 2 and matte painting. A number of visual effects companies worked on Tron Legacy, not just Digital Domain. I was one of a few artists using Terragen at one of the other companies. Our 3D clouds were rendered with standalone Terragen 2, although I made a few enhancements to address particular needs (most of which will be included in the next public release).
The
Ray trace everything option in Terragen 2.1 was used to render clouds with true 3D motion blur and to efficiently render the holdouts of solid objects (buildings and terrain). The geometry for these holdouts was simply imported using the built-in OBJ Reader. In most cases the same models used by the Lighting and Rendering artists were used "as is" without any particular modification for Terragen to render clean holdouts.
The company has developed a pipeline for publishing cameras to Terragen, but it is via the Nuke .chan file format which we've talked about in other threads. Two cameras for each shot, as this is a stereo 3D film.
So, we only worked on a portion of the movie, but it's still quite a lot of Terragen-rendered atmosphere to look forward to

While I'm at it, there is
another movie coming out next year with lots of Terragen 2 environment in it. Trailers are already online, but I won't say any more just yet!
Matt