I think that MS will try to lead off the next generation again. There won't be a clear winner between PS3 and 360 this gen, and the only two advantages 360 really has over PS3 is an earlier release and an aggressive courtship of developers. MS will want to get that early-release advantage again next gen, and I also think that X360's hardware will start to seem old before PS3's will, mostly because the 360 controller is the most old-fashioned this gen.
I think that Sony will be interested in squeezing as much as they can out of the hardware they've sunk so much money into (R&D, marketing and loss-leading) before they're willing to invest another truckload of cash into the next generation.
Wii will ride the honeymoon of the victor this generation and last at least 6 years. They'll probably release their next console whenever they feel they've perfected their next control scheme, which may or may not come before Sony has their PS4 out.

"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.