Sony and Nintendo stay for sure for at least 2 more generations I say.
I'd guarentee Microsoft has at least 1 more generation and it is dependant on how the "720" performs for if theres a "1080".
It wouldn't come as a shock to me if Apple enters the gaming market with an iStation or iConsole. It has the iPod, iPhone, Laptops, Desktop computers, the ony thing left is a Console.
Next Gen will be:
Playstation 4
Wii 2
Xbox 720
The next Gen after that could be:
iConsole (apple system)
PS5
Wii 3 (or a different name)
Xbox 1080