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I am curious about this because it uses the internet sharing option which is forbidden and could terminate your ISP. But it uses WLAN from you Airport to broadcast the signal thru your Powerbook/iBook/MB/MPB/MP/iMac. The guide is from here but I want to make sure it's safe before I use this option instead of wireless router.

http://guides.macrumors.com/Connecting_Mac_with_Wii

You're doing the same thing with that, effectively, as you are with a wireless router. No ISP will actually terminate your service for using multiple computers on one connection.
 
Nearly every ISP will not terminate, or even try and check (I don't think you even can) if you are connecting with multiple computers.

Also, if you are using the internet sharing option instead of an airport base station, you need to use the longer password WEP standard as the Wii is not compatible with the way OS X sends out the password. I think it is a 128-bit password you need.
 
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