Contrary to our update that said the Wii does not have a sunlight problem, a company rep just said that the sensor bar in fact does have interference issues.
Nintendo told Kikizo today that this is a known issue with the Wii. A company representative explained that the Wii sensor bar is affected by head-on exposure to sunlight, which could cause "some interference". But he likened it to not being able to see a TV with the sun shining straight on it.
So yes, you're going to have problems in direct head-on (apply directly to your Wii) exposure to sunlight, but that will probably be rare unless you can't find a way to close your blinds at certain times of day.
Image courtesy Stuff (NSFW)