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:

(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:

see the project page here: