Portage 1.9.1 'emerge --pretend --emptytree world' does not produce the expected output. It seems to be equivalent to 'emerge --pretend --emptytree system' ie: It does not appear to look at the world file to include packages that were installed after the initial 'emerge system'. See bug 1897 for a related --emptytree problem.
OK, this happens because world profile entries are ignored for packages that aren't installed. And since --emptytree tells emerge to pretend that no packages are installed, the "world" file is basically ignored. Investigating some options here.
Will be fixed in Portage 2.0.2