Apparently there  has been some confusion in the 'sphere the past few days on whether 32-bit based PCs running Windows Vista will support HD formats including HD-DVD and Blu-Ray.  HD-DVD and Blu-Ray support will be made available in Vista via ISV applications and according to a statement by Adam Anderson, nothing in any version of Windows Vista would make that determination. Here's the official Microsoft statement on the topic:

It is up to the ISVs providing playback solutions to determine whether the intended playback environment, including environments with a 32-bit CPU, meets the performance requirements to allow high-definition playback while supporting the guidelines set forth by the content owners.

No version of Windows Vista will make a determination as to whether any given piece of content should play back or not.

I'm  working to get additional detail on the topic and will post more here when I can.