"Like all of the graphics in IE9, canvas is hardware accelerated through Windows and the GPU. Hardware accelerated canvas support in IE9 illustrates the power of native HTML5 in a browser." [1]
They have a demo linked on that page where I get 1 fps when I run in it chrome. Their screenshot of IE9 running the demo shows 60 fps. I suppose hardware can make a difference, but thats its a really impressive.
I just ran few of my canvas demos in IE9 preview and while being pretty fast, they didn't seem to be HW accelerated.
Performance seems to be roughly on the Firefox level, which is less than Chrome and Opera (all current browsers do software rendering AFAIK).
Anyways, great news, web just got richer. Now let's start our prayers for WebGL ...