My edition of
screen on Debian (package version 4.1.0~20120320) has lock functionality built-in; hitting
C-a x locks the entire environment (so that not even detach with
C-a d is permitted). The Debian version of the man page for screen writes, "Call a screenlock program (/local/bin/lck or /usr/bin/lock or a builtin if no other is available)."
If you want to lock a terminal session you can use
vlock. From the man page, "vlock is a program to lock one or more sessions on the Linux console."
I also tried installing
vlock and running
LOCKPRG=$(type -p vlock) screen. After hitting
C-a x to lock the screen I saw that
screen was correctly calling
vlock to lock the screen. Again, the entire environment was locked so I couldn't suspend screen with