Microsoft has made available the official Windows 7 Upgrade chart, via allthingsd which indicates which previous versions of Windows are available for an in-place upgrade to Windows 7, and to which version they can upgrade.
You can view the chart below:
As you can see from just a glance at the chart, it's relatively confusing. Windows 7 offers no direct upgrade path from XP, which was announced very early on in the Windows 7 development life cycle. Very specific upgrade paths from other versions of Windows Vista exist also. You can perform a "Custom Install" which is pretty much a clean install, but you have the option to install Windows 7 to another disk without overwriting data.
Bloggers from around the web, such as Ed Bott of ZDNet and Joe Wilcox of Betanews, have lashed out at the chart, pointing out that there are a number of flaws in the chart. He points out that it is over simplified, and that for users who want to upgrade from 64-bit flavors of Windows, it will be a completely different story.
Ed has created his own version of the chart, which can be found here, which makes the chart easier to understand, and reduces the chart size by half, to 14 cells.
Windows 7 will be generally available on October 22nd, and is now available on Technet and MSDN for subscribers.