About a year ago, I remember responding to a question from a colleague concerning Flex in comparison to HTML5 by saying that HTML5 was not ready yet. For the most part, I still hold that opinion but with additional qualifications.
We are currently bidding on a job that requires iPad as the target device and another that needs a private label application that will work on the desktop, iPhone and eventually Android and Blackberry. After logging many hours researching objective-c and HTML5/CSS3 my first thoughts were that all development needed to be done in objective-c. But, after viewing some of the other videos from Apple, MIT, and Stanford, and doing some small tests, I think that most applications for the iPad/iPhone can be implemented completely in HTML5/CSS3. So here is my revised/updated response...
If you have complete control over the target web browser (i.e., webkit/khtml based) then HTML5 and CSS3 can't be beat. Using the 'canvas' tag/object enables drawing anything that you can draw with flex/flash and also includes drawing text (not possible with flex). Using CSS3 enables all the transitions that are currently available in flex including easing methods. Using WebSocket, provides peer-to-peer real time communications. So if your target includes Safari, iPhone, iPad, iPod/Touch, Google Chrome, Opera, Firefox, as well as the Android, Sprint and Blackberry browsers, the new technology is ready today.
The missing piece is IE, currently at 70% of desktop browser installations. An application that needs to run only in a standard client browser is still better off using Flex (95% penetration) until Microsoft complies with the latest W3C specs. But, the flash player will probably never make it to small devices (it's a hog), so if the requirement is to run on desktop and phone, especially with touch gestures, HTML5/CSS3 is probably a better choice. The down side is waiting for Microsoft to catch up with the fully developed world and hoping they will fully comply with the W3C.