That's the date for the final formal specification, which is a document. HTML5 has always been developed in tandem with browser makers who add features as they go. With IE9, all major browsers will have canvas support regardless of the formal specification status. So the web can be using it all through that time.
It's possible the HTML5 spec will change before then, but it's unlikely to be a serious "breaking change" - and even if it is, we only need to tweak our javascript runtime to fix it. It's nothing for end-users to worry about, I don't think.