Sencha has done a great job making ExtJS a successful product and I'm sure they'll give some interesting implicit inputs about where Flex can improve from their perspective. I see this as a great opportunity to understand how they justify the transition to potential enterprise customers. They have done the job for us already so it's worth taking the time to listen and take note.

"if you know your enemies and know yourself, you will not be imperiled in a hundred battles" - Sun Tzu

If you want to register:

http://pages.sencha.com/flex-to-sencha-webinar.html?ls=sencha_twitter&ss=Flex2SenchaWebinar

Once you register to the webminar they offer the "The Development Manager’s Quick Guide to HTML5" for download:

http://pages.sencha.com/rs/sencha/images/Sencha-WP-Dev-Mgrs-Guide-to-HTML5.pdf Which seems to be a good overview of what they are going to cover.

Cheers!