It is true that the IE browsers seems to be the slowest in adopting the evolving HTML5 standards although their latest has made headway. There is a bit of light around this cloud however. Google has something called ChromeFrame http://www.google.com/chromeframe that can modernize the older IE browsers so they can understand HTML5.
Noone can say that IE doesn't work with HTML5 and noone can say that IE works with HTML5. HTML5 it's not a one big tool, it's a toolset of different objects and properties that provide new/awesome functionality to our browsers. IE9-IE10 supports a lot of those new HTML5 features. To found out if your browser supports one of HTML5 features you can make manual test or use special lib for example as Modernizr for tests. Also there are special libs to make your old browser (I think even ie6) a better one, so it will understand new html5 tags and even some js functionality.
HTML5 is designed to be backwards compatible as possible. Every valid HTML4 document is also a valid HTML5 document, so in that sense IE supports HTML5. What people usually mean when they say "Internet Explorer doesn't work with HTML5" is one of two things.
Commonly they mean that IE8 doesn't support the new HTML5 elements such as <article> and <footer>. Since these elements have no actual functionality defining whether they're supported or not is a bit of a non-issue apart from the fact that IE8 and earlier have a bug which prevents any CSS being applied to an unknown element. There are well know workarounds for this (and it's covered in chapter 1 of Hello! HTML5 and CSS3).
The other thing they may mean is that IE doesn't support all HTML5 features, and that's true, but it's also true for every other browser. There is great competition among browser vendors today to get all the new stuff implemented (and this is a good thing).
CSS3 is also backwards compatible in most cases. Valid CSS2 is also valid CSS3. It is often possible to us jQuery or a similar library to add classes in the markup in browsers, such as IE8, which don't have good support for the new stuff in CSS3 (and this is discussed briefly in chapter 7 of Hello! HTML5 and CSS3).
Yes. I don't believe we'll see wide adoption of IE10 until the Windows 7 version is released. There'll still be a significant user base of IE8 and IE9 users, especially inside corporate firewall, until Windows XP and Vista fade into history.
Having said that, although IE8's HTML5 support is poor compared to current Chrome, Firefox and Opera, it does still have some support for HTML5.