Add to DemonPrincess' advice, in that the direction of the normals might not be conforming. Check all the normals, and you might have to reverse/conform them. It's happened to me a few times-- and there is a noticeable lighting contrast,
http://www.passthepress.com/?p=557 Its all "I cannot confirm or deny" at this stage, but that generally translates to "yes, this is true, but I'm not allowed to say."