It's still too early although the current gen has just about reached a point where developers are starting to have to rely upon experience with the current hardware to continue to really push the boundaries of the spec limitations to optimize their game engines and resources. That being said, there's still a lot left to be done with either the 360 or the PS3 before the hardware platform becomes the real limitation in producing specific projects beyond just lowered resolutions and reduced frame rates.
2-3 years seems like the bare minimum for the next gen to debut so I'm guessing 2011-2012 at the earliest, most likely later.
If anyone were to jump the gun first, I'd probably guess it would be Nintendo since they could easily produce a lower cost HD system with hardware above current specs of the Xbox or PS3 and sell it at a profit from the beginning. As it stands, the hardware restrictions of the Wii has already made it non-viable as a development platform for a majority of the current gen hit franchises. Up to date specs on Nintendo's next console would allow developers to give it proper versions of all those franchises it's currently missing out on.
Unless Natal was really intended for the Xbox 3, it doesn't seem to make a great deal of sense for MS to go through the marketing effort of relaunching the Xbox 360 as a "new" console once it begins being bundled with Natal next year. Two years after re-release would seem to be the bare minimum if MS expects to market Natal Xbox 360 to mass market consumers which is still pushing it if they opt to release a new console two years later.
SCE has the most to work with from a hardware perspective, and really just needs to keep concentrating on continued 3rd party support, SCE studio games, continued development and expansion of PSN and returning to profitability before they even bother announcing another PlayStation.