Read the full article on Josh Bancroft's blog, on short it's about the lack of WDDM drivers for Intel 915 integrated graphics much needed to run Vista Aero, Movie Maker and DVD Maker and the reson for that is that the Windows Vista WDDM driver spec came after 915 was complete and in production and is missing a hardware piece needed to be WDDM compliant. Some people complained that Aero used to work on 915 in one of the release candidates, and Josh explains that Intel didn't change the drivers but Microsoft has changed the product to not allow XPDM drivers access to those features.
Also found this on Intel's website explaining the hardware limitations in 915 chip.
Q5: Why doesn’t the Intel 915 Express chipset family support Windows Vista Aero?
To be honest from reading Intel's explainations I kinda get that it would've worked but would've been slow. I think there's a bunch of politics involved as well, they can't release some half compliant WDDM driver since they are such a big company and obviously they don't want to spend time and money on an older chipset so they've dug up these technical details. Fair enough, but they should've came out with these explanations from the beginning and most people probably would've understood, now there's a bunch of angry people out there feeling that they've been deceived, they are not sure now if it's Microsoft or Intel or both.
Personally, I don't think they did a great job with the XPDM driver either, I can notice is slower on my R2H in Vista compared to XP even when drawing up windows and not themed ones - so maybe they should've tried to put together some half-cooked WDDM driver and release it through some Intel community channel, there are you are boys, we couldn't build a public release WDDM driver cause Microsoft would never sign it, why don't you have a go at this version but keep in mind you are on your own, don't come back crying if it brakes. It would've been installed by the 6000 technology geeks petitioning now, there you go is slow as we told you a while ago, then everyone would've disabled Aero cause it draws too much battery but happy now they have a WDDM driver :-) and maybe then I could've been able to change the brigthness from Vista Mobility Center :-) but since that won't happen, back to the drawing board on that one...