Very interesting project. Waiting for somebody clever enough to work this all out and tell us how it is done exactly.
Most laptops take about 90W-120W for the whole system.
Say the card takes 200W + 120W = 320W for a system mining with one 5870 or 6950 etc.
I'd like to see you doing that with a normal desktop !
My whole rig with 2x6950 and A8-3850 draws 400-420W. Total hashing power 800MH/s... Measured from wall socket.
Once you get two cards or more running off of a single machine, full desktop rigs do become more reasonable. I am, for example, running just a single non-reference 5870, overclocked to 999 MHz, memory underclocked to 150 MHz, and getting a hashrate of 437 MH/s, off a netbook. That gives me an average draw of about 225W from the wall socket. My numbers give basically the same hashrate to wattage efficiency that you're getting with your two card desktop rig. If you were only running one card in your desktop, you'd be less efficient than mine, but then you also have more expandability.
It really comes down to how much people want to spend on their initial hardware. With my setup, I still have the capability to upgrade to a 5970 before needing to move up to any kind of multi-card setup. I'm obviously at the low end of the hashrate totem pole here with my initial investment some months ago, but I wanted one of the most efficient one card rigs from the get go.