This is a great contribution - thank you for your effort and your support on this board.
I have a wee undocumented feature - as noted by ogg2k:
Enter a tracking number without making any other change on your orders page, click update, and you get "Warning: Nothing to change. The order was not updated.".
I realise that testing in this case doesn't equal reality and we'd be changing the status while entering the tracking number so all would be well. But I thought that you may be a clever enough bunny to iron that out sometime between eating / sleeping / real job.
Also - to solve the error
"Warning: Cannot modify header information - headers already sent"
you need to trim the white space before <?php and after ?> in that file.