Can you cancel a Steam preorder?

I've never bought anything from Steam before, so pardon my inexperience.I'm not sure if I'm going to get Left 4 Dead yet, but I'm definitely going to give the demo a try. However, rather than wait til it's made available to the public on the 11th, could I preorder it today and then cancel that preorder before the game's release if I don't like the demo? In other words, does Valve bill you when you initially place the order or when you download the software, and can you get your money back/prevent them from charging you if you decide to back out before release?Can you cancel a Steam preorder?
When you pre-order off of Steam you buy the full game not just a down-payment like $5 or so, though you can get a refund and the game will be taken off your account I believe.though this would be better asked on the Steam forums to make sure. Can you cancel a Steam preorder?
I believe you CAN get a refund, but I wouldn't count on it as its not a policy they have to follow.Honestly, why not wait for the demo and if you like it them just pre-order it. The early demo is more for the die-hard that know they are going to buy it.