Did you ever figure this one out?

1. Check your materials. Is it invisible? Try duplicating the model and deleting all materials, then render. This will tell you whether or not it is a materials problem.

2. Check your normals. Has it been flipped? If all the normals are calculated inside, that would make your model invisible. 

3. Check your layers. Is your camera on the same layer as the eyeball? Tell your camera to render all layers just to make sure.

4. Check your model. Does it even exist? Create a blank Blender file and append your model, then render. 

 

Thats all I can think of right now let us know !