In a letter sent to Microsoft customers titled 'An Update on the Windows Roadmap', company senior vice president Bill Veghte states:
You've also let us know you don't want to face the kinds of incompatibility challenges with the next version of Windows you might have experienced early with Windows Vista. As a result, our approach with Windows 7 is to build off the same core architecture as Windows Vista so the investments you and our partners have made in Windows Vista will continue to pay off with Windows 7. Our goal is to ensure the migration process from Windows Vista to Windows 7 is straightforward.
Contrary to popular belief, Microsoft never officially 'slipped' Windows Vista. Many off-hand remarks were made concerning the operating system's release date, but once the date was officially set, they didn't release it a day late. Assuming this letter can be seen as an official communiqué, this means we have Windows 7 in our hands in 18 months.