Steam users now have the ability to cancel pre-orders without having to contact Valve, according to reports on the Steam Users' Forums.
We took the screenshot below, and as you can see, Steam's transaction history now includes a "Refund an item" link for pre-orders. Until this point, Steam users who wanted to cancel a pre-order had to open a support ticket with Valve.
Valve charges your method of payment when you place an order, even for pre-orders, and it's unclear if refunds for pre-orders only put money back into your Steam Wallet or return the purchase price to your method of payment. We've reached out to Valve for more details, and will update this article with any information we receive.