I have a couple findings from the Wii Maintenance Mode. Here is what changes when you activate it:
-Wii Message Mode disabled
-Internet disabled
-Certain settings can't be changed.
-Turns off after activating any channels.
Channel Preview Screen changes:
Disc Channel: Nothing changes here.
Mii Channel: Nothing changes.
Photo Channel (1.1): The only thing that shows up is the bulletin board backround.
Wii Shop Channel (1.1): Nothing.
News Channel: No message/news appears.
Forcast Channel: No message/current weather appears.
Nintendo Channel: The logo doesn't appear.
Mario Kart Channel: The Mario Kart is in English, but, the Channel is in Japanese. Also, the karts show up like this:
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Everybody Votes Channel: Nothing.
WiiWare/Virtual Console: Nothing.
Check Mii Out Channel: Nothin.
Internet Channel: You should know this by now.