Sorry if this was already dealt with and my search-fu failed. The icons are showing up fine on links that go off-site, but going to admin/config/user-interface/extlink spits up:
Recoverable fatal error: Argument 1 passed to drupal_attributes() must be an array, string given, called in [...]/includes/theme.inc on line 1515 and defined in drupal_attributes() (line 2222 of [...]/includes/common.inc).
Checked Apache error logs and found nothing relevant.
If there isn't a fix I've managed to miss I'll try to have a crack at it tomorrow, if I can work out where to start looking! :)
It was working fine in alpha7 by the way, just didn't seem to like my beta1.