All modern browsers support HTML5. IE does not because it's not a modern browser. IE9 does but, again, it is not a modern browser.
To add to this, HTML5 has not yet got any frozen official standard, so no browsers have the full HTML5 support. But all modern browsers like - Safari, Chrome, Firefox, Opera, and Internet Explorers (later to version 7) are adding new HTML5 features along with their new releases. So if you target audience is a bit technical or those who don't upgrade their systems... you are good to go.
Yeah worst part of it is when you consider the percentage of users still using IE, although most have updated to IE8 you would be supprised just how many havent. 1 of my sites 70% of the visitors are using IE.
No. IE<9 will NEVER have HTML5 support and what you see in IE9 is all you will get till IE10 comes out in, what, 3 years?