In light of the recent billing fiasco (Accounts set to re-up on April 20 & 21st just plain didn't and then the phone lines to the billing department died. Niiiiiice. Has since been resolved though.), well I figured I should get off my bum and post about my game card.
I had posted previously that by buying a gamecard at Amazon.com you could play for $12.50 a month. I ordered my 60 day Game Cardand added it to my account.
Bottom line - It was Fast and Easy.
Once you log into your account look in the list of links in the upper right hand corner for the "Add a Game Card" link. The instructions there tell you to scratch off the grey strip on the back and type in the code that you find. I liked this feature because that way if I wanted to buy a card off of someone I could verify it had not yet been used if that strip was still there.
Once it's added you get a success message like this:
More information about using your game card can be found here at the Blizzard Gamecard Billing Information page.