I have modified my install of Bit.ly Shortlinks to create j.mp URLs instead of the default Bit.ly URLs. This was a simple change to the API request (including the &domain=j.mp querystring argument), and is working just fine.
I would like to see this be an official option, supported by an admin screen setting, or by a wp-config.php variable declaration if admin screens aren't planned near-term.
Thanks!
m@