TWRoO on 26 March 2008
The light only flashes for new messages, so anything sent by one of your Wii friends, or by Nintendo (be it an update, important message or something from the Mii contest channel)
It doesn't flash for the news/weather channels, those are updated regularly but your Wii wont flash when it does that.
You can dim or turn the light off in Wii settings.
