Hi! I just localized this plugin to norwegian, and I discovered a few localization bugs..
1: the load_plugin_textdomain (line 29) needs to be changed if the plugin is to be placed in /mu-plugins
2: some strings were not localized
3: some strings were not in the pot-file.

I have made suggestions for corrections and uploaded the php file here:
http://no-wp.googlecode.com/svn/plugins/wordpress-mu-domain-mapping/tags/0.5.2/domain_mapping.php

(with this mod, the language files have to be in the folder wp-content/plugins/wordpress-mu-domain-mapping/languages)

and the norwegian language files here:
http://no-wp.googlecode.com/svn/plugins/wordpress-mu-domain-mapping/tags/0.5.2/languages/

see the project page here:
http://code.google.com/p/no-wp/source/browse/plugins/wordpress-mu-domain-mapping/