In my file "/var/cache/edb/world" I have a line =nvidia-kernel/nvidia-kernel-1.0.3123-r2 to fix the version of nvidia (necessary for frequent lock-ups of the screen). If I do now > emerge --deep --update world the nvidia-kernel is updated to the latest version, irrespective of the contents of the file "/var/cache/edb/world" I am using > portage --version Portage 2.0.49-r20 (default-x86-1.4, gcc-3.2.3, glibc-2.3.2-r9, 2.4.24-ck1)
the world file does not use depend atoms (no prefixes/versions) if you want to lock in a file, use the files in /etc/portage (see portage(5))