We ran into an issue at the office today, and I wanted to document the solution for my own future reference. Our firm has architects and engineers in-house, and each discipline typically works in separate models that are then linked into each other. This particular project was being done in the 2012 versions of Revit MEP, Revit Structure and Revit Architecture. The electrical engineers found that they were not able to place face-based light fixture families in their model, and host them to the ceilings that were modeled in the linked architectural model. Reflected ceiling plan views were being used and the ceilings were visible, but the fixtures would only host to the Walls.
It turned out that the Ceiling model category was turned off in the affected views in the Electrical model. This did not prevent the linked architectural model from displaying the ceilings, and they were also TAB-selectable in the Electrical model. But it did prevent Revit MEP from "seeing" them as a potential host. Once the ceiling category in the electrical model was turned back on in those views, Revit was able to find the ceilings and host fixtures to them. It was necessary to check the Show categories from all disciplines toggle in order to do this.