Upgrading from 2.0.51.22-r3 to 2.0.53_rc3 or above and trying to do an emerge -p world shows and empty list of packages to emerge. However, downgrading again shows that such pkgs exist indeed. I'm quite sure that the world file is found in the correct position and read, since I was asked to perform a "emaint -f world" to fix it. Opening it with an editor of course shows me the complete list. Maybe related to bug #76568? I cannot find that file anymore. Please note it's an old bug not active since 20050301.
*** This bug has been marked as a duplicate of 108271 ***