According to the latest roadmap from Microsoft, it would appear that Windows 8 will arrive sometime in 2012. This would follow a three year product cycle that would keep it in line with Vista and Windows 7 releases.
The road map is clear about the targeted date and does not try to hide the fact that the code name is "Windows 8". Little else is known about the OS other than its targeted date of release.
Let the speculation begin.
Image courtesy of msftkitchen.com