The release dates were first unveiled by Ina Fried, and later confirmed by Ed Bott. This probably makes it the first Windows version to be released ahead of schedule, putting an end (for now) to the various jokes about new Windows versions always being released behind schedule. Fun fact: Windows 7 will be released almost exactly 8 years after Windows XP, which was released October 25, 2001.
Microsoft also confirmed that there will be a guarantee program under which people who buy computers with Windows Vista close to the launch date will be eligible for a free or discounted copy of Windows 7. Details are unknown as of yet, but as with previous incarnations of this program, computer makers will decide pricing information.
Looking back at the history of Windows releases, having a development period of roughly three years is consistent with the past. The jump from Windows XP to Windows Vista obviously took the longest (5 years), although the jump from Windows 3.0 to Windows 95 actually took two months longer, but only if we disregard Windows 3.1 and 3.11 for Workgroups.
In any case, this should give everyone some idea on when to plan the possible purchase of a new computer.