If you would like to mock up functionality in your mashup application, there are some tools which can allow you to do this fairly easily. You can even make the prototypes directly in the phone.
Popapp lets you take photos of paper sketches and create a linked, multi-page prototype that you can click through. This is great for quick turn-around user testing on lofi models.
Marvel App lets you take .psd designs from your dropbox and add clickable areas to create multi-page click models from hifi designs. It will even sync with the files in dropbox so when you make changes to the design the prototype also updates.
These tools were suggested to me by Anton Stupnev.