After years of requests, it appears Valve have finally begun to offer refunds, at least for preorders. As spotted by donham of SPUF, any game that has been preordered through Steam now has a refund option.
The refund button may be found in a user’s transaction history, as seen in the screenshot below.
Currently the exact details of the system aren’t known however. Presumably when a game is refunded, the amount spent on the purchase is added to the user’s Steam Wallet.