Not a mobile web, merely a 320px-wide one

- Many devices don't support pinch to zoom in/out.

- You can't assume the resolution of the screen. Just aiming for the resolution of an iPhone isn't sufficient.

- iPads, iPhones/iTouch are going to be sometimes held in portrait vs. landscape. The user shouldn't have to turn the screen. Either should be ok.

- Kindle browser is black-and-white, and scrolling and zooming is exceptionally difficult. (Not that many are trying to use it, yet.)

Not to mention that sites need to be designed for accessibility...

There are so many things to consider, and farming out the work to someone that specializes in this sort of thing is really the only way to go. Every organization can't afford to have every type of device on-hand, nor the expertise to know what works and what doesn't.