I've just made a fresh install of WordPress 3.4.2 and it seems that this simple but usefull plugin doesn't work anymore.
When I define login and API key in wp-config.php, then "Get shortlink" disapear from the editing screen (it's a not a custom post type).
I tried both logins the human readable one, and the other one that looks like to a password with a mix of characters and numbers.
Even tried to reset my API key...
Too sad, it was so much usefull and simple :(