I pre-ordered with GP, but decided to pick a Nexus 7 up at my local Sam's Club this weekend instead. Since I hadnt been charged yet, or received any shipping info, I stuck out the 49 minute wait to cancel my 16GB order (Ordered on July 6th).
The lady was very nice (I kinda felt sorry for her 'cause I know they were slammed with calls) and she informed that my order was "past the point of no return". (ie. that it could not be canceled through Google). The representative recommended that I either refuse the UPS order (and wait for my card to be credited) or call my credit card company and get them to block the 2nd authorization. After all, if they cant charge it, they wont send it.
I decided to take the 2nd route, called my credit card company and they put a temporary (1 month) hold on any transactions from "Google Devices". That way, I dont have to wait on money to get credited back to my account (which I am worried might have been as big of a mess as the shipping/charging has been).
I wish you GP guys (& gals) the best of luck!