On the other hand, I have always been against using Flash for design and here's why. I think Flash is good for videos and applications but I hate it when the user is locked inside nested viewports. Ideally, you should never see more than one scrollbar in each direction. Although, you can do Flash right. For example see how Picnik expands to fill the who browser view-port. But most developer don't bother and that's a big downside for me.
As for HTML5, I have to say I am a huge fan of both the definition and the multimedia features. But I don't kill it will kill Flash, and here's why:
1. While you can create very cool applications using Ajax, Flash still provides richer, more desktop-like applications. For example, there are some pretty cool image editors written in Ajax, but those that are written using Flash give you a better desktop image editor experience.
2. There is no standard codec for video that everyone can agree on. Maybe if Microsoft didn't exists, everyone else could agree on a standard and HTML5 could replace Flash for multimedia. But that's not gonna happen, is it? While both Google and Mozilla support the open-source VP8 codec, Microsoft has already stated IE9 will only support the H.264 codec. Now that is still being debated, but I'm not getting my hopes up.
You would think with the popularity of MP3, things would be better for audio. But according to W3Schools' HTML5 audio tutorial only Chrome and Safari support mp3 in HTML5 at the moment! Also note that IE9 is the only browser that doesn't support any of those codecs at all!
If things go on the way they are right now, we are back to the old days of multimedia where you had to support Real player, Windows media and Quicktime! If fact, that's the whole reason Flash got so popular for videos in the first place... Back then you would tell users to download the appropriate application, but now first you have to explain what a codec is and then tell them to install it!
Interesting enough, while I was writing this article, YouTube posted this!
I also recommend reading this Tech World article.