Off and on since I got my wii I have been having problems with the cursor being jumpy and had even had a couple dead spots on my screen. I was affraid it was a defective wii. I soon put two and two together and realized most of the problems were occuring on the side of the tv next to my lit christmas tree. Turned the tree off and the problem went away.
I then did a little research and discovered that the "sensor bar" is not a sensor at all. It is an infrared light source. The actual sensor is in the wiimotes, and the wiimotes tell the wii where they are pointed by finding the light source. Therefore any potential source or reflector of infrared light can mess up the remotes so they don't know where the tv is. Things like candles, mirrors, and apparently christmas trees can affect the performance of your on-screen pointer.
There is little information about this problem, and Nintendo has said little to nothing about it. I can only assume Nintendo's silence is due to the fact it is the nature of the beast, and there is no fix for it other than be conscious of decorations in the room that can create infrared background noise.
I don't know if there is already a thread about this... I didn't see one, but I appologize if this is a repeat.