This thread is going way OT, if you want to talk about the mining business or profitability this thread is not for you.
I want to understand instead the likelihood and impact on Bitcoin of getting a large number of gamers to turn on their GPUs and start mining. What if CoinLab's efforts lead to hundreds of thousands of additional people opting to cash small amounts of BTC out of their system?
First of all, Valve would never do this, but IF
they build in a BTC miner into their steam client that ran on low priority while you weren't gaming, and stopped when you fired up a 3D intensive program, then they let you buy games/hats/etc with your balance, that would catapult BitCoin into the stratosphere.
Actually, if I were Valve and I controlled the Steam client, I wouldn't mention BTC at all (bad for bitcoin), I'd rename the miner something like "The Piston". It would run as described, when the user isn't in a 3D application, and it would mine BTC for me, while the user would earn "PSI" points, equivalent to 10 PSI per $ or something, they could then spend those on select
digital goods that cost me next to nothing to make (hats n stuff).
The electricity bill would piss some people off, but imagine having the whole Steam community mining for you like a huge farm...