Click on "Cancel Items" (the text) instead of the Cancel Order button. Then Submit. Worked for me! I have a 256 silver ATT reserved for pickup at lunch time today! I couldn't stand the wait when I know I can get it today!
I hit Cancel and received a notification that my Order can't be canceled. I am guessing it is probably preparing to ship but my order info hasn't been updated. I feel like half a fool for waking up at 3:00am, not getting a launch day phone, and now not being able to walk into my local store to get the exact phone I want while they sit there on the shelves. Smh.