I'm having this odd issue on one of my meshes, where the lightmap is all checkered, and not nicely smoothed. I've been searching the UDN knowledge base and whatnot, and several forum posts, and tried the following fixes:
-Encase the level in a BSP brush room, so the light traces don't have to travel far.
-Re-import the mesh that's giving issues.
-Re-do the smoothing groups.
-Add a proper collission mesh.
-Make the mesh one-sided. (as it was two sided before).
The mesh has no volume. It's a plane, with no backside, and no actual interior. I'm talking about the curved wall that also acts as the ceiling as it goes up. Check the picture below. I'm using a Pointlight to light the room, which is located inside the small gap in the wall, combined with a Movable Pointlight, also in the same location. That gap is part of the same mesh. This is Production quality light.