IS there any mobile site that eases answering questions whilst I'm on the iPhone. It is quite difficult doing it on the iPhone with the regular site.
If not is there an API I can use to code my own version?
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No, there is no mobile optimized stackoverflow site.
No, the API is currently read only.
If you wanted to pursue coding your own, I'd suggest writing a website wrapper in C#, Python, PHP, etc, hosting it on another website, where the wrapper would replace the default css with a custom version you design.
It isn't trivial due to the extent of the design of the site, but it may be your best bet.
The (beta) mobile theme has been rolled out to all sites on Stack Exchange, you should be able to see it on your mobile devices.