On the WebGL mailing list someone mentioned that it's not HW accelerated except on Mac OSX which makes hard to see this as a fallback for WebGL. There is also talk of using flash(molehill) as a fallback but that entails some conversion of things like shaders and sharing the data between the javascript vm and flash plugin etc.
EDIT: also for IE, it's probably best to just get Chrome Frames.
EDIT: also for IE, it's probably best to just get Chrome Frames.