Nintendo, Sony, MS...they can bleat about the lifetimes of their consoles all they want.
The reality of the situation is that advances in graphics and such are being made EVERY day, and at a quicker rate, so todays consoles will all look horrendous in about 3 years time...
Sonys initial idea for the PS3 (not having it based in a single piece of hardware, more it being 'downloaded' to your tv') will be the future to avoid new games consoles every half decade