The OSX devs have implemented their own way of setting up the $PATH which is different from all other Unices as far as I know. They use a program called path_helper which will read /etc/paths and the files in /etc/path.d and modify the user's default $PATH accordingly.
According to this, the $PATH is only set this way for login shells. This is probably why iTerminal starts login shells by default (I've always wondered why they chose that).
According to the same source, path_helper is run from /etc/profile and /etc/csh.login which should affect
- bash
bashlogin shells - csh
cshlogin shells - zsh
zshlogin shells (should read /etc/profile/etc/profile) - all other shells that source
/etc/profilewhich should be most login shells.