Currently Online 
Wii-2: november 2011. Why? Well, Ninty actually knows what happens when they decide to pull out a console far after its competitions. Under normal circumstances (if the Wii was on par with PS360) it could be even a year later, but I think that's the max time Ninty will be able to milk the last SD console. It will be stronger than PS3 (about two times) but still will be a weaker console compared to its concurrence, not so much like the Wii compared to PS360 though.
X720: late 2012. Why? MS actually did a good job releasing a console earlier than Sony and so grabbed a huge number of early 7th gen PC tiles like Oblivion or FEAR, for instance. The most safe move would be doing the same again but development costs and the need of a really noticeable difference between 7th and 8th gen will hold the strings of a early launch.
PS4: late 2012. Not much to say, I just concluded they will follow the same path as MS. It will be a weaker and cheaper system than X720.
My predictions lol
