I kind of like this idea but it seems to me it already exists in a couple of ways. Pre-ordering software is a community investment action where you buy the software before its actually completed, i.e. Windows 7. And the second way is buying stock in the company itself if the company offers that option.
But all that aside, Im not sure why Valve is even looking for upfront investments in their future projects. They have been so successful with their products and with Steam you wouldn't think they were hurting financially. You would think they have all the money they need to continue producing great games. TF2 is one of the coolest games ever made in my opinion and the way they keep adding to it has just made it better.
Maybe a better idea would be simply to flesh out the core or bare bones of a game, sell it for $10 real cheap and then keep implementing ideas as you go and selling those updates for a nominal fee. Again, the way TF2 has evolved is very cool. If they had waited to release the game until it was all the way finished, meaning in the current state that it is today, the development time would have been much longer and the needed funds would not have been available.