I've always been able to cancel pre-orders a day before the actual release date (orange box) via amazon.com, but I've never tried ON the release date. Many offline personal horror stories about gamespot preorders make me stick with other retailers. Amazon.com's policy on pre-orders is that if there is drop in price before release date, you will pay for the lower price. I'm not sure about the increase in price, though. Amazon's shipping is usually pretty reliable and immediate, but some people online have been reporting slowdowns.
Steam is probably going to do a 10% discount + beta, so I might go with that. I'm not as concerned with owning the physical copy as everything has been shifted to Steam. I'll make my own backup after downloading and write onto a DVD-R or external HDD.
edit: I'm a bit surprised that the game is probably going to MSRP for $40 (PC) and $60 (X360). But it is valve, after all, catering to their strongest fan base and development platform. So there is a minuscule chance that this might be a price mistake, so doing a pre-order isn't going to take your money away forever unless you allow it to during release day.