In the attached file I have a material named Bldg Front. It is assigned to a vertical plane. It uses a diffuse map of the facade of a building. A UVW Map is applied to the plane and channel 1 is used. The gizmo for the diffuse map fits the plane as seen below.

A second UVW Map modifier is assigned to the plane. Channel 2 is assigned to it. Channels allow multiple UVW maps to be assigned to the same object so that different mapping parameters can be assigned to different components of the object. I created a jpg image named Half Black that has its top half black and bottom half white. I made the gizmo for the cutout map much larger than the plane as can be seen in the image below. This ensures that the entire plane can be covered by either the black or white portion of Half Black.
The animation starts with this gizmo's top half (black) covering the plane making it invisible. As the animation proceeds, the gizmo is keyed to move up thus the white portion begins to cover the diffuse map making it visible. In the attached Max file the building is totally invisible in frame 0 and totally visible in frame 40. The image below was captured at frame 35 so the building is almost totally revealed.

lee.minardi