Also, lost in this discussion is the fact that one of the more popular browsers doesn't support HTML 5 in its current version or possibly even its next.
Not within ogg/theora it doesn't because there's no way of signaling the correct amount of data to buffer as used in the buffering mechanisms of modern video codecs.
a) Excanvas is slow. I've used it in some projects. Its not too bad, but performance was an issue for me
b) There are other parts of HTML5 that IE doesn't support. Although I guess someone will eventually create similar shims to make <video> work as well as webworkers and database access.
Not natively, but http://excanvas.sourceforge.net/
Does streaming video work under HTML 5 specs?
Yes.