Cancelling a preorder made via Steam previously required you to contact the company to get things started. It looks like this is no longer the case, with users now reporting that the cancellation process can be initiated via a link on the Store Transactions page of your account.
It's not clear whether the refund is delivered to your Steam Wallet or your credit card / PayPal account (with the latter options being more desirable), but providing users with a way to cancel a preorder themselves is definitely a step in the right direction.
Steam client now offers self use refunds for pre-orders. [Steam forums, via ValveTime]