The could probably backport D2D but a lot of those older XP machines have lousy GPUs anyways. But you make a good point about XP being sold in 2009. I think they should have made a non-D2D version of IE9 for XP. There are a lot of users that only upgrade their browser through Windows update.
It will be more than a version check, they'll make use of the.01% difference. But they could easily do a version check to rem out the Win7 features.
They're trying to encourage OS upgrades since that is where the money is. I'm not saying it is a good strategy, I can understand not wanting to support XP but with Vista they are going to piss a lot of customers off. I doubt it will be worth the negative PR.
You are absolutely right that web developers need to get tough with them. Don't even give them the option of viewing a broken website, use the IE6update script or autoforward them to the IE8 homepage. Web devs need to take more responsibility.
I don't care if you bash them for that, I was just explaining in another post how D2D and DX10 could have been backported to XP. Some of us use what works and don't take sides as if the software world is some epic star wars battle.