Sorry for the very late reply, but a lot of people visit this page because they are searching for the same. This is exactly what you are looking for:
http://userscripts-mirror.org/scripts/show/171425.html (old link: http://userscripts.org/scripts/show/171425)
IT IS FINALLY HERE!
This script allows you to install all your STEAM games without user intervention.
Made by the one and only real Master-Guy
How it works:
1) Log in into Steam on the computer where you want to download the games.
2) Install this script on any computer, it may be the same one, or a different one. I used the same computer myself.
3) Log in into the Steam Community in FireFox
4) Go to the following URL in FireFox, but replace the asterix (*) with your Steam Community name: http://steamcommunity.com/id/*/games?tab=all#installall
The script will wait 5 seconds before starting, and will trigger all game installation with 10 second intervals.
The build-in download scheduler of Steam will pause the updates while starting the new one, and will only download one at a time.
This is to optimize your download speed.
This installed around 400 games and 150 DLC's onto my computer in 8 hours of time without any further user intervention.
It might however require you to start the script twice if the internet connection isn't optimal.
If the 5 seconds isn't enough for the complete page to load, change the following line:
var timeBeforeFirstGame = 5000;
Increase the 5000 to the number of milliseconds you want to wait for the page to load.