It's not a zero-sum game with native vs web apps. The web will not kill native apps, but for many applications the web offers a better platform than native development. For example, look at the rise of GMail's web client over MS Outlook - for many users a web client is easier, with mail accessible from any device with a browser.

New HTML5 features will extend these advantages to more types of apps beyond webmail. The same question came up over at Quora, with thoughful responses: http://www.quora.com/Is-HTML5-the-mobile-app-killer