Based on Nintendo's previous release history, it seems unlikely that we will see their next console prior to seeing their next hand held. They won't release them within the same year. If they did, it would be a new play for them.
So I'd use news of their next hand held to gauge about when we can expect to see another console.
Second, I don't believe the release of another console means that the Wii will be discontinued. As long as it continues to sell, Nintendo will keep it in production, even if it means adopting a two tier console plan like Sony. By then, it's not unreasonable for Nintendo to be able to sell them for $99, which will continue to move units.
As for Sony, most seem to think that because the PS3 has been oft quoted as having a "10 year life span" that they have no plan for releasing a new console prior to 2016. But consider the life span of the PS1 before production finally stopped: December 1994 through March 2006. Eleven years plus.
PS2 from March 2000 to current, with no sign of being cut from production. March 2010 doesn't seem like a big stretch to keep it in production since it still generates profits.
PS3 from November 2006 to November 2016, going by Sony's estimates, with a new system being introduced most likely six years into its life span in 2012, based upon past release trends. Still at least four years from now, but that is far from being eight years away.