So I have my Mac game.
What would the main measure I would need to take on if I wanted to export it to...
Wii Xbox Live and iPhone
?
By the way, Wii exporting is to WiiWare right?
Well you have to make sure you have the licenses to them. You can buy iOS licenses online, but if you wanted to get a Wii or Xbox license you have to 1) be a Wii/Xbox developer already and 2) contact Unity's sales department.
Apart from getting the license to develop for these devices, the biggest issues you will face will be:
a) performance. The devices you want to target will have very different horsepower and so you'll need to spend time optimising the geometry, shaders and textures to suit.
b) input. These devices have very different input behaviour. The consoles use controllers, the iPhone is touch based, and I suspect your Mac game uses a keyboard and mouse.