Download and install the iPhone theme
Once the file is downloaded, unzip it and double-click on iphone.rwtheme. This will launch RapidWeaver if it is not already open and automatically install the new theme. You will be prompted to quit and restart RapidWeaver to make the new theme available.
Disclaimer (to the extent needed given the obvious pre-alpha stage of the theme): my iPhone theme is still very much in its infancy and is made available for testing purposes only. It is provided as is. Use at your own risk ;-) You can build upon it by following the very few publicly available guidelines on web development for the iPhone. Beginners like me can also find guidance in the RapidWeaver SDK.