I have a model of a building with a sidewalk around it. Both show up when I switch to rendering mode in both Blender Render and Cycles Render mode. But: when I hit F12 for a final proper render, the building shows up -- but the sidewalk doesn't.
I've checked the normals and I thought the normals seemed pointed in the right direction. I've set the Clipping on the Camera to a high number. I removed all the materials from the sidewalk and added a new material-- the problem persists. I've exported the blend as an FBX file and the Sidewalk shows up fine in Unity. Just in final renders, it doesn't show up.
Here is a blend file: http://www.pasteall.org/blend/35247. I tried to destroy everything unrelated to the issue and label things in a clear way.