Mike, From what I’ve seen, there can be benefits to restructuring information on a mobile device.
The main reason is that you want to see *less* information on a mobile device. Filtering some content can actually improve the user experience.
Of course, this could be done with CSS and “display: none;” but that means you’re sending a lot of structural markup that is unused (and wasting bandwidth on a device that doesn’t have much.) The alternative of forking your information structure and maintaining two versions of a site is also unattractive.
The best course of action is to evaluate development on a case-by-case basis: know the costs and find the solution that fits best.
As far as the user agent is concerned, the iPhone specifies this platform information: “(iPhone; U; CPU like Mac OS X; en)”. There’s more detail in the “iPhone guidelines”:http://developer.apple.com/iphone/designingcontent.html
As others have pointed out, you shouldn’t place too much weight on that string, since there are other mobile devices that can benefit from handheld markup and styling.