Soundwave on 17 September 2009
I think Nintendo will launch the Wii successor (whatever you want to call it -- Wii HD, Super Wii, Wii 2, etc.) in 2011 ... basically keeping that 5 year cycle.
But it will be a much more "phased" transition, meaning the "classic Wii" will be sold alongside the new model and you may even have cross-platform games (ie: Dragon Quest X runs on classic Wii just fine, but on Wii 2, it runs in HD resolution) at the beginning.
