Sony is the only contender that really doesn't need to seriously look into releasing new hardware in the next two years or so. The Cell processor still has a ton of upside, as long as people are willing to learn, and push the intentionally difficult architecture. This console still hasn't reached it's potential, and with Blu-Ray technology, it can pack a ton of data onto it's discs, which helps future proof it a bit. Look what the PS3 has already done, and you can see that right now, it's in position where it can last for a while, unless one of it's major competitors does something so outlandishly awesome that Sony is forced to react defensively by dropping the PS3's price and immediately working on PS4, but that would require a very dire situation.
The Xbox 360 is almost tapped out in hardware terms, if not already, considering it's about 7 year old hardware. Lets face it, the Blu-Ray is kicking the old DVD9's ass twelve ways to Sunday, it's time for a new media format for Microsoft's consoles. Either take blu-ray, or really develop for the next step, but no matter how you slice it, DVD9 standard has got to go, it's past due. Maybe this time, though, they won't be in such a hurry to release a console earlier than the competition that they'll have time to really test out the new prototypes to prevent anything like RROD again. Also, going full on 1080i will be desired.
If Nintendo has taught us anything, it's that being unique and family friendly can sell a ton of consoles. It's not so bad to play by different rules than all the other kids. But if Nintendo has learned anything, it's that gamecube hardware won't keep people interested forever. They may or may not keep the motion controls, but they know deep down they've got to upgrade their graphical prowess, else their uniqueness will be a moot point. Lets be honest now. Isn't it sad that they'll be releasing a portable 3DS that has greater processing potential than their flagship console? I thought so. Miyamoto may swear to dance to a different drum, but not even he can't deny that there are people who want cleaner, smoother, and higher overall graphical quality. I personally want a Legend of Zelda in HD.
Because PC's are so plug and play, they'll always have cutting edge potential, so no worries there. I had to include it simply to round the 4 out.