This doesn't come up all that often, but - say you had your Wii plugged into a TV that supports progressive scan and had that enabled. Then you switch TVs to one that doesn't. Think something like bringing the Wii over to a friend's house. This only applies when using the Component cables (red, green, and blue video plugs) - the Composite cable (single yellow plug) doesn't support 480p and the Wii won't try it. Which is why the following works.
The PS3 is nice enough to provide a feature where holding down the power button resets it to 480i. The Wii does not.