Question about Vista, DX9 and DX10

[MSIC]

I've just been wondering about something.
I read with all the nVidia 8800 launches about DX10 that DX10 will only be available for use for machines running Vista (well, under Windows anyway, we'll see if Linux does it). Maybe DX10 games or apps will run in some sort of backwards compatability DX9 type thing under XP, but Vista seems to be true DX10. Ok, I accept that.

So why is it that Vista's graphical front end is DX9?

OK, i understand that not everyone is going to have DX10 hardware in their machine. Probably 99% of all new machines sold will be DX9, and that's not counting the people with current hardware who upgrade their OS.
So that makes sense.

But DX10 has less overheads, and from what I understand can do the same much more effiiciently. So again - why not have a 'true' DX10 mode for effieciency, and a general DX9 mode for compatability?

I know I dont fully understand the processes involved, but have I missed something here or would this in fact be more logical? Especially when we think of laptops batteries that are going to be drained just from running the desktop...

[charleski]

Because they were developing DX10 at the same time they were writing Aero. Sensibly, they realised that it would be a bad idea to base the entire front-end of the OS on a technology that hadn't been fully implemented yet. They can always update Aero to DX10 in a future service pack when the wrinkles have been ironed out of DX10.

[TheAnimus]

Also, when developing aero, they knew that the DX team was working on a system for providing dx9 under dx10, that would provide great performance boosts.

With the new directx 9l regular dx9 games will run faster if you have dx10 hardware.