Microsoft is supporting SVG in IE9, and gave a detailed explanation of why they were doing it on the IE blog:
http://blogs.msdn.com/b/ie/archive/2010/03/18/svg-in-ie9-roadmap.aspx
SVG's main advantage is that it becomes part of the DOM, so you can use CSS to style it and javascript to modify it. Canvas, by contrast, must redraw every frame completely. This makes canvas suited to spectrum analyzers, video processing, fast-paced games, and other non-gradual animations. SVG is better suited for gradual animations.
As far as 3D is concerned, the future is WebGL, a thin shim over OpenGL ES, but it's far off. Microsoft has not committed to supporting it, and that means it's not going to be in IE9. Maybe IE10, maybe not.
If you do use SVG, I recommend using svgweb to abstract away the browser differences (falls back to a flash applet on outdated browsers).