If I know how to model in a CAD program like Pro/E, how hard would it be for me to learn Blender, Max, or Maya? I'm a programmer and I only need to know enough to make placeholder models (I'll get a real modeler for my project later), but still I don't want to use cubes for everything lol!

Modelling in CAD programs is nothing like modelling in true 3D programs, having done both. You really need to relearn the entire workflow, really, but to do 'placeholder' models isn't difficult.

it shouldn't be that hard. I started with autocad and now use 3ds max. I'm learn maya so changing to modeling should be slightly easier