I must have dozed off at the wheel because this was new to me:

Finally, Indigo is different from the other new Longhorn technologies in an important way: It will run on some older versions of Windows. The plan of record is to offer Indigo on Windows XP and Windows Server 2003, as well as on the Longhorn release of Windows. This is important because it means that Indigo-based applications can interoperate not just with those on non-Windows operating systems, but also with applications built on some older versions of Windows itself. For a technology meant to improve connections among applications, this is a good thing.

From David Chappel's Indigo: The end of the rainbow.