A little while ago Nintendo revised their privacy policy stating that they can indeed update your Wii system without notifying you. Actually, I'm not so sure if they update your system, as I believe they put it something like "check for and remove unlicensed software" or something.
EDIT: Beaten by an edit. Here are the goods from Stevey's link:
"We may without notifying you, download updates, patches, upgrades and similar software to your Wii Console and may disable unauthorized or illegal software placed on your Wii Console to ensure that your Wii Console is operating properly and efficiently, comply with applicable laws, assist law enforcement, protect us and our customers, or prevent the use and distribution of software obtained through improper channels."