If you buy a machine preloaded with Windows Vista after July 1st 2009, you will be eligible for a free upgrade to Windows 7 once it's released. There are a number of restrictions, though; you are not allowed to upgrade from one language to another. In addition, the only upgrade paths allowed within the program are from Vista Home Premium to Windows 7 Home Premium, Business to Professional, and Ultimate to Ultimate. All other paths are not allowed; people who buy Vista Starter or Vista Home Basic are not eligible for upgrades, for instance.
The dates are not final yet, so they are subject to change. What they do confirm, however, is that we will see at least four versions of Windows 7; the ones mentioned above, and an enterprise edition.
In other words, for those of us who had hoped for a simple home/professional divide, forget it.