how about integrating with Role Weights to provide the role ordering?
parts of role weights are now in D7 core. the rest is being ported to D7 in the same module.
the module could pick up the default ordering from role weights.
if the admin checks a checkbox, custom reordering would be provided (as is now)
sorry, posted on wrong module
Drupal is a registered trademark of Dries Buytaert.