Great!
I am actually going to do something like this.
The problem at the moment is that I don't know anything about programming or the arduino things.
For now I am testing out some rgb led strip that I got from ebay to see if it's worth ordering more of. I guess the lifetime of these led strips isn't as good as the ones I have seen at our wholesalers. Also the RGB amplifiers from ebay are a lot cheaper than the quality ones here. I wonder if this will result in a burnt down house 
Anyway... Iam going to do something similar to this:
http://fibreled.jet.ie/wp-content/uploads/2009/08/IMG_9826.jpg
I will be using this controller(common anode) when not watching tv:
http://www.ledlightsworld.com/24-keys-infrared-rgb-controller-p-120.html
And when watching tv I'll use it as ambilight with Arduino.
All video and all audio is coming from a computer(Mac).
As my lights will be in the ceiling I don't need more than one rgb out channel for the strip.
I noticed that a lot of people are using solutions that only work with specific software like winamp, vlc or xbmc.
Which isn't good for me as plugins usually, IF they come for mac are updated much later than the ones for PC.
Thats why I like yours better as it will work with anything(right?).
Someone on youtube did it like this:
"A very simple hack using lowpass filters on the VGA color signals and an Arduino that generates PWM from the filtered signals driving an IKEA "Dioder" LED strip."
What do you think of this?
You wrote "I skip every alternate pixel making my program 4 times faster".
How about every 4 or 8 pixels?
What happens? A feeling is that you still would get a ok average when watching movies?
You also wrote "delay(10); //delay for safety" and "delay(10); //just to be safe"
Is this milliseconds? Could you explain what the safety is for?
Is it possible to reduce or increase the overall brightness of the leds if they for example are too bright?
Is it possible to make the screenshots smaller than the actual screen to get better performance?
If you cut XXX pixels off every side of the screen and still get a good average reading?
Is it much more complicated and more expensive to use Bluetooth instead of a USB cable?
I would like to have all equipment on the other side of the room.
Thank you
C