Well, what I think it really means is that M$ unified DirectX support with a framework and some higher-level rendering engines into a cross-platform system.
Overall, not MUCH of a change, but I imagine it means you'll be able to port between platforms without touching a line of code (just recompile to the new platform).
The Xbox XDK uses DirectX, but it's not a transparent port between Windows and Xbox. The missing pieces lay in the framework and what's possible in the APIs. M$ did something similar with WindowsCE AutoPC support - they brought back the GUI APIs and MFC into CE, making it easier to port apps.
Overall think of this as DirectX 10+. It will be a real winner if it incorporates a decent 3-D engine like Lithtech's.