Remove the black circles and the white triangle vanishes.
We actually see the Kanizsa Triangle Optical Illusion often in everyday life. I was cruising the web months back and came across a blog that presented the below figured used by "The Limited" chain of stores in their windows. He correctly pointed out that below we see a "Kanizsa Girl", we all see a girl where no girl actually exists.
We have all seen this technique used in marketing from time to time. If anyone comes across the blog that made this observation please leave me a message so I can give him credit for noting this observation.
Update: Adam C. pointed out another example of this type of illusion used in advertising. In the below FedEx logo we see an arrow which isn't really there. You don't really think that the arrow is in their logo by accident do you? Thanks Adam.