Contrary to earlier jumps to conclusions, the Wii Sensor bar doesn't really have a problem with the sunlight directly shining onto it. Sure, one of their earlier engineering feats was getting it to work in sunlight, not to mention UV light, IR, and florescent lights. But Nintendo now says:
Our testing thus far shows no great risk of light interference when playing a game that relies on the pointer and sensor bar.
No problem here. Good job Japanese engineers! May you go out and get some sun once the Wii launch is complete.