This is an issue that has impacted my thought process recently. I've been considering getting a new mobile phone (PDA), and as a long time Palm user I'm interested in the Pre. The OS looks great, it has a physical keyboard, and the device is fairly open (you get root access with the developer SDK). However, in the fragmented mobile market, the iPhone is the king. The development effort and ecosystem around the iPhone is amazing. In a fragmented market, I'm not sure I want to bet on a niche player.
There are many reasons for this fragmentation, some technical, some institutional. I'd like to see more of the institutional ones removed (no exclusive carrier agreements, more 'open' devices), but on 'limited' devices, tailoring the experience to the hardware is important for usability. There is a reason that the iPhone SDK was the key to the devices explosion. Apps delivered in the iPhone browser are just not as compelling.
To achieve a mobile web as ubiquitous as the world wide web, we need to make the browser the default application delivery platform, just as it is on 'real' computers. I'm just not sure the devices and networks are there yet.
