Well, flash is a proprietary framework that really does slow things down. I can't remember the last time I watched a flash movie without it freezing the video part way thru and having to catch up.
As for HTML5, it's not really a video format or a shell format it is a web standard...the most current web standard. The iPad will be able to view any h.264 video that is embedded in the browser. I checked my site and it played wonderfully right on the site.
Most flash video sites are using H.264 but delivering thru a flash player. With HTML5 you are just presenting the video by way of embedding as has always been done. iWeb is already compliant, so if you made your site using it any videos already work. The only thing that is needed is the codec to play it...on Apple systems (mac and mobile) its already there...on windows it needs to be installed (what else is new) on android it just doesn't work.
Apple is trying to actually use a standard rather than a proprietary plugin and people jump down their throats...lol, it's funny. If adobe made an all flash OS that didn't support web standards people would be pissed about that as well!