The zune method isn't going to work because 8862 isn't on nokia's servers yet. I check navifirm everyday and they deleted the original 7.8 files and all that's on there now is 7.5.
Why? If you are waiting for ATT, they haven't even put it up on their servers yet. Even when they do it will be days or weeks afterwards till it shows through zune. The fastest way is using Nokia Care Suite and Navifirm.