Then again, on the desktop, as this Technology Review story points out, HTML5 will allow for assembly and mashups of images, including video, with databases to create, as the story notes, a personalized image of someone running through a neighborhood when you put in an address. We can imagine all kinds of database mashups and functionality done on the fly to create new application-like experiences in an HTML5-compliant browser. We can assume the same kinds of functionality will come to mobile devices.
True, there is the nascent Chrome OS, and we could see the day when Safari has the functionality of Objective C. But for now, I'd say, Joe has a point for mobile devices. Not that HTML5 won't take hold, and not that a lot of functions will be handled by browsers on mobile devices, and not that you shouldn't have a good mobile site if a lot of your user base accesses your site via mobile platforms. But don't expect to be able to give them all the functionality through the browser that you can through an app. (And, as an exec at Ad Mob pointed out, you can sometimes access HTML5 browser functions through an app, anyway.)