Obviously the Wii version is gonna be SD, but the HD consoles have a SD mode also, i've said a lot that devs can do the development first on an HD console, take the SD mode and work with that on the Wii adding the proper controls... They don't need to dumb down the game like they did with Dead Rising...
I don't think PS3/360 games render in SD, even when the console is in SD mode. What probably happens is that the image gets downscaled to SD from the native resolution (which btw is not even HD for many games).
Anyway, changing the resolution is the easiest part. To convert a game to the Wii they have to reduce texture/model quality, remake shaders (since the Wii's GPU works quite differently from PS3/360 GPUs, which are basically like PC graphics cards). And probably more stuff. They also have to reduce RAM usage (though reducing graphics quality should already help a lot with that), and of course fix any excessive CPU usage.
It can be quite a lot of work depending on the games. But it's obviously possible for many games.