I'm not very familiar with html 5, but if it can cover pretty much anything flash can, I'd be happy to switch to html 5.
Ever since flash player 10 was released my cpu started running at 100% everytime I open a website contain even the smallest flash app. If it's the aim of the adobe flash developers to create a heavy layer to create animations to make your site a little more attractive, then I'm out!
I'm pretty sure though that html 5 won't be able to replace everything flash can. Just one thing that comes to mind is sockets.