Using ARD, use the Curtain option to control the computer. That will ask for a "Lock" message, that will be displayed while you are controlling the computer. The display will be changed to a picture of a "Lock", and your message... So be polite.
Otherwise, you can take control of the computer normally, and click on the "Lock" message.
If you are using the built-in remote control software with Leopard / Snow Leopard, there is no method to lock the screen while controlling it. There had been some functionality in 10.5.0 - 10.5.5, but Apple removed it in 10.5.5. (It was unadvertised and required a hack to enable it)
DISPLAY and AUTHORITY
An X program needs two pieces of information in order to connect to an X display. (Note that wmctrl is an X program, even if it accesses other processes' windows rather than creating its own.)
It needs the address of the display, which is typically :0 when you're logged in locally or :10, :11, etc. when you're logged in remotely (but the number can change depending on how many X connections are active). The address of the display is normally indicated in the DISPLAY environment variable.
It needs the password for the display. X display passwords are called magic cookies. Magic cookies are not specified directly: they are always stored in X authority files, which are a collection of records of the form “display :42 has cookie 123456”. The X authority file is normally indicated in the XAUTHORITY environment variable. If $XAUTHORITY is not set, programs use ~/.Xauthority.
Inside a screen session, the environment variables are determined when the session starts, unless you explicitly change them at some point. So if you start a screen session locally on your desktop machine, then attach to that session remotely, $DISPLAY and $XAUTHORITY are still pointing to your desktop machine. But if you start the screen session from an ssh connection from some machine C to your desktop machine, then the variables are not set. (They would be set to point to C if you had an X server on C and had enabled X forwarding over the ssh session.)
Getting the values of the variables
As far as I understand, you're trying to act on the windows that are displayed on your desktop. If you're the only person using your desktop machine, it's very likely that the display name is :0. Finding the location of the X authority file is harder (under the default setup in Ubuntu, it's in a file with a randomly generated name).
Here are a few ways to obtain the values of DISPLAY and XAUTHORITY:
The easy solution is to always start a screen session from your desktop, perhaps automatically in your login scripts (from ~/.profile; but do it only if logging in under X: test if DISPLAY is set to a value beginning with : (that should cover all the cases you're likely to encounter)). In ~/.profile:
case $DISPLAY in
:*) screen -S local -d -m;;
esac
In the ssh session:
screen -d -r local
You could also save the values of DISPLAY and XAUTHORITY in a file and recall the values. In ~/.profile:
case $DISPLAY in
:*) export | grep -E ' (DISPLAY|XAUTHORITY)=' >~/.local-display-coordinates.sh;;
esac
In the ssh session:
. ~/.local-display-coordinates.sh
screen
You could detect the values of DISPLAY and XAUTHORITY from a running process. This is harder to automate. You have to figure out the PID of a process that's connected to the display you want to work on, then get the environment variables from /proc/$pid/environ (eval export $(</proc/$pid/environ tr \\0 \\n | grep -E '^(DISPLAY|XAUTHORITY)=')¹).
Copying the cookies
Another approach (following a suggestion by Arrowmaster is to not try to obtain the value of $XAUTHORITY in the ssh session, but instead to make the X session copy its cookies into ~/.Xauthority. Since the cookies are generated each time you log in, it's not a problem if you keep stale values in ~/.Xauthority.
There can be a security issue if your home directory is accessible over NFS or other network file system that allows remote administrators to view its contents. They'd still need to connect to your machine somehow, unless you've enabled X TCP connections (Debian has them off by default). So for most people, this either does not apply (no NFS) or is not a problem (no X TCP connections).
To copy cookies when you log into your desktop X session, add the following lines to ~/.xprofile or ~/.profile (or some other script that is read when you log in):
case :$DISPLAY:$XAUTHORITY in
:*:?*) XAUTHORITY=~/.Xauthority xauth merge "$XAUTHORITY";;
esac
Then inside screen you'll only have to setenv DISPLAY :0 (or whatever the display number is, but it's likely to be :0 as explained above).
¹ In principle this lacks proper quoting, but in this specific instance $DISPLAY and $XAUTHORITY won't contain any shell metacharacter.
Best Answer
A vnc server doesn't have to be attached to a physical display. My recommendation is to start a vnc server on :1 on your remote Linux box and connect to that.