For me you just have to look at the motive for these companies to release new consoles:
NIntendo - They generally seem to do so when they feel their current system is reaching the end of it's lifespan. 3DS has been timed alongside DS software sales sliding, piracy going up and so they can differentiate their games from the emerging iPhone market. Wii sales are still going very strong and there's plenty of price to cut, so I don't think we'll see this too soon.
Sony - The console normally comes as part of a company wide initiative - it's no coincidence the PS3 was built with HD, to sell HD TV's, picture capabilities, to sell Sony cameras, Blu-ray, to sell Sony Blu-ray etc - and also built with the foresignt to sell 3D stuff later on etc. The PS4 will come, and be based around, whatever Sony wants their future company wide push to be - I have no idea what that is but HD Tv's came out 8 years before the PS3 and Blu-Ray players 2 years, so I'd say we're a long way off a PS4.
Microsoft- I'm a bit more iffy on their motives, but I think the 360 was done simply as the Xbox wasn't the massive hit they hoped and they wanted a leg up on Sony. The 360 however is doing pretty dam well for them, and I doubt they'll be willing to interfere with that, or cut Kinect's long term life short, with a new console.
For me Nintendo do currently hold the cards with regards to the new gen, but they face alot of challenges in repeating their success of the WIi. First and foremost is the question of what will it's ace be - the 3DS has pretty dam amazing graphics for a handheld, but glasses-free 3D is it's big wow factor. The Wii will no doubt match 360 and PS3 for graphics, but just like the 3DS I don't think it's core graphics will be what Nintendo sell it on, and until we know the next console wow factor I don't think we can really judge whats going to happen at all.