Dang, that's a bummer. I hadn't thought of that, nor did I check. The dropdown only shows the domains listed in the Domains -> Manage Domains list.
Your two choices are: do a redirect, which isn't that great, or re-install Drupal as your domain. If you've already set up your Drupal nicely, see if there's a Database Dump/Backup utility in the Admin menu. Create a new Drupal, then dump from your old install and restore to your new install.