I'm a late and reluctant convert to the use of HTML5 to generate web applications. I still believe it's an immature technology. I imagine the real test will come when Facebook releases their new HTML5-based interface (not released at the time of this writing). If there are holes in HTML5 capabilities, the folks at Facebook are going to run into them trying to support a wide variety of devices from a single code base and implementing a less than trivial interface.
Well, the jury is in, and there are more indication that Facebook is abandoning it's HTML5 application and reverting back to native applications for mobile devices (at least for iOS).