I had a little discussion about this after my announcement on the Tactical Command forums. The conclusion is that CAD and digital renders are allowed, but the final images that will be submitted for voting must be photographs of a physical 3D print. This should be fairer alongside the photos of putty and clay sculpts, and should better determine if the render works at 28mm.
It follows that the render has to go through a 3D printing company like Shapeways or Printapart, meaning extra expense and a wait for delivery. So if you want to try this particular medium, and think you can stand the bite out of your potential winnings, get started now. :) I'm told that Shapeways prints can take 2-3 weeks to arrive. It may be prudent to spend the first month sculpting and allow the next month (or most of it) for delivery.
Also, it'll help your entry photos and possible casting if the print is as smooth and clean as possible. You can order the print in cheaper materials but it may take more time and effort to defuzz the surface. From what I've seen, Shapeways' Frosted Ultra Detail (FUD) is a popular choice, with minimal cleanup needed.