I just found a crack in my T520 laptop case ...on the keyboard side in the lower left corner right below where they put the "Intel Inside", "Windows 7" and "Lenovo Enhanced Experience" stickers.
I also have a crack in the exact same place. Have any of you got any further pursuing this with warranty? like you and others in this thread I had owned my T* Thinkpad for only a matter of weeks before the crack appeared. I have had no luck in my first contact with IBM NL, they claim it is wear and tear or physical damage and not covered. I have been very careful with my laptop so was stunned to see the crack. I am sure this was not from ' excessive force' on my part.