Microsoft has long allowed people, including businesses, to install an older OS using a newer OS's license and consider it valid. However, the process has never been easy for OEMs, and it has been a particular bone of contention with Windows Vista. In the current system, each machine to be downgraded requires a support call to Microsoft and a frustrating series of steps that makes it awkward at best to perform a downgrade.
They have responded to the angry cries of their customers, however, along with large demand to opt for XP over Vista. Thus, they are simplifying the process, making it an easier option and one that can be done on a large scale.
The new system will allow OEMs to submit large batches of licenses to be downgraded. Unfortunately, they are only giving this privilege to its top partners about 170 or so. Regardless, they still hammer on about how the launch of Vista has just been a huge success when many are saying otherwise. Anything they can do to help their own customers frustrations is a good thing.