I never did it but was told that you inserted a small chip which acted as a digital switch to change the setting depending on how you booted the Gamecube.
You are probably right, but it really wouldn't make sense. Or does anyone seriously think that someone who wants to buy a PS-3 and suddenly find a Wii in a store will suddenly decide to better buy a Wii instead? It would be much cheaper, but it simply does not offer the features of a PS-3 or even the Xbox 360 (there you could at least buy an additional HD DVD drive to wqatch HD Movies and it wold only cost 50 Euro more (less than the price of one game).
On the other hand: Nobody in its sane mind would think to buy a PS-3, that costs more than two times more, when he only wanted a Wii. What kind of features have these two consoles in common, that the Xbox 360 does not have?