The PS3 slim is perfectly capable of running OtherOS, it's just Sony took it out of the system's software (or never put it in, rather), so they've been dealing with 2 different software versions ever since the Slim was released.
Edit - I don't think Nintendo would ever do that, and it would only need to be done via a software update, seeing as the Wii hardware *is* the GameCube hardware (essentially). I guess they would save a few pennies by removing the Memory Card/Controller ports, but other than that, there would be no reason, or benefit for doing so.