First off, dont even try arguing that the average Wii is used the same amount as the average PS360 - If you do you are in denial. From personal experience I am sure the average Wii console is used far less often the average PS360 console, so do you think this helps out the Wii's failure rate?
Those Wii's out there that are used very often seem to cope well, so there's no denying that its a quality piece of hardware, but with a lot of Wii's not getting played that often surely this is bound to help its failure rate?