Out of what little we know of the next version of Windows, this feature might just be the most interesting yet. A team at Microsoft is hiring developers to work on adding native support in Windows 7 for Virtual Hard Disks (VHD) - Microsoft's semi-proprietary specification for single-file virtual machine hard disks. This has rather interesting implications for IT administrators and even home users.
For example, having an VHD dedicated to gaming with optimized system configurations is entirely feasible then. A dynamic VHD would mean it would only take up as much room as it needs, you could move the file on many system and have the same experience, but best of all, you can still enjoy the maximum native performance at the same time being able to load it as a virtual machine to maintain and configure without rebooting.
Considering how much we don't know about Windows 7, this is extremely promising.