When you really step into it and start thinking about Nintendo expanding their market and bringing in new customers that doesn't necessarily correlate to Sony or MS due to the simplicity of the target demographic of the Wii.
So with that in mind how can that reflect on Sony & MS?
First off, these new consumers that Nintendo is bringing in by these simple games can lead to them becoming a more avid gamer in which they could end up being interested in what the PS3 & 360 have to offer. Thus expanding the core gaming market for all three. For the ones that don't go beyond those simplistic casual games of the Wii they will stand alone until Sony & MS are able to match that of the Wii.
Now the question is how many of these new consumers will covert? The way things look, not that many, given the majority of the games we will see on the Wii will be those simple games. Sure we will see games for the core gamer but no more then the PS3 & 360.
So in essence by Nintendo expanding their market doesn't mean it will have a big effect on expanding the market for the PS3 & 360. In which the pie is only getting bigger for Nintendo.
Time will only tell if the PS3 will be able to survive on a much smaller market share.