IE7 and IE8 have not shifted any huge numbers from IE6.. if you look at the statistics for IE, and Windows, you see that IE6 pretty much matches up to the amount of XP users.. web developers don't think aw god nother browser to worry about.. web developers want to generally stay in a happy middle ground where we can use modern enough features, and perhaps stick our necks out of newer features when its sensible to do so.
IE6 is uniquely the only browser web developers REALLY worried about, it is SO FAR behind todays browsers you stand little chance of making a modern site that can be tweaked to work ok in IE6.. any other modern browsers may disagree on small things, but these are things that are generally easy to fix..
If you are including IE6 support in your site then the approach you take to building the html/css is COMPLETELY different, you try to avoid anything at all complicated.
I'd also like to point out the immense level of piracy that XP received, to the point today people expect to pick it up for nothing on a torrent site, I'd say theres a noticable percentage of that XP base that are still there purely because they are of the false expectation that an OS should be FREE to them, and are finding it impossible to successfully pirate newer versions of windows.
These are certainly not people I care to support, nor should microsoft, nor should microsoft after two versions of windows be trying to 'make happy' the people that * and moan the newer revisions aren't up to scratch.. they will NEVER make you happy because your expectations for what an OS should be are unrealistic and twisted, and completely the opposite of any vision microsoft has.
So in short you can get screwed, stick to your OS that will see increasingly smaller product releases/updates, or move to a new OS, whatever OS you want.
Ofc Microsoft won't say this, they have to keep up the PR and smile and convince you to upgrade to their latest OS, but again expect that to become increasingly 'old' for them to keep up.