I first heard the comment of "HTML replacing Flash" from Steve Jobs. (He hates Flash perhaps because it crashes on his Mac computers I'm not sure. I think he's out of touch. The proof is his ridiculous product the "iPad".)
The point is HOW GOOD IS IT FOR DEVELOPERS? Answer: not as good as Flash. Yet. Why do I say this with my limited experience? It's all that "try... catch" compatibility code you need in Javascript to work in all the browsers. WHAT A LOAD OF RUBBISH. ActionScript/Flash is ONE piece of code/app for ALL browsers.
The other thing Flash has is graphical (and non-graphical) development software tools to streamline the process.
So THE SHORT ANSWER IS THAT ADOBE WON'T LET HTML5 "replace" flash, at least for the short term. But it can compete with Flash if people make enough development tools for it particularly WYSIWYG graphical tools, like if Google develop an easy to use WYSIWYG editor for making GWT apps. And that's a big IF.
