Microsoft is going after the bells the whistles, rather than a functional OS. They are trying to be Apple and trying to impress the masses with "look how cute is this system". IE9 is not different.
Windows 7 offers many improvements over XP. The WDDM driver model alone made siginificant leaps and bounds for hardware and device support to remain consistent and available. The interface, streamline in performance and updated gui are all nice as well.
There is absolutely NO reason to hinder Windows Vista and Windows 7 users by fragmenting their future web browser to support a de-supported 10+ year old OS anymore.
I'm pretty sure Firefox 3 will be the last firefox to run officially on XP. Chrome 7 may not support XP. Webkit is going to try and compete against IE9 in which case the WDDM driver system of Vista or 7 will be a requirement.
The web & Microsoft are moving beyond what was acceptable 10 years ago in an OS.. you can chose to stay behind, but please.. please.. please stop blaiming MS, Vista or Windows 7 for problems it does not have :)
Windows 7 is a HUGE upgrade over XP and a streamline of what they attempted to do with Vista. The kernel is better, Multi Core support is better, Multi GPU support is better, the UI is cleaner & more consistent, the deployment, upgrade & support functionality is so much better, the WDDM driver system is easier to develop for, less reboots are necessary, no problems with the UAC alerts like before so on and so forth.
Newegg has upgrade for 79 bucks (oem), MS sells the family pack again for 3 upgradeable licenses for 120-140 or so and there are many other options available as well. (The option to not even run XP or IE9)