hehe, we will find ourselfes in the same situation as we were some years ago: all browsers will interpret html5 differently. not even the basic html/css is interpreted the same way on each browser TODAY. thats why a plugin totally makes sense.
and i'm looking forward for scribd for launching their html5 player because then i get the sourcecode completely for free directly from their website :P
for magazines on iPad html5 is not an option because it natively can display pdf pages. and having a webview (browser) for rendering html5 on a mobile device is a real bad idea.