With the ability of HTML5 web apps to store data locally and have the browser pass along the device’s hardware states, I believe that HTML5 wbe apps can do almost anything you could want an internet-using application to do. The only exception I can think of is high performance 3D graphics, as even great 2D graphics are possible with CSS3 and HTML5′s canvas tag. Already there are nice libraries like RaphaëlJS that work in the iPhone’s Mobile Safari.