Sketchkit includes pre-drawn iOS interface elements that can be used to create wireframes for iPhone apps.
*Yes, Keynote! It's surprisingly awesome at this stuff.
Android coming soon
Wireframes are meant to explore the purpose and functionality of a product. Wireframes are not supposed to imitate the final user interface. In fact, laying too much importance on how the product will look while wireframing can be distracting for the researcher and misleading for the client.
Sketchkit attempts to use a design that makes it clear that these prototypes are not meant to illustrate the design direction for the user interface.
I gave a talk about this - you can check my slides and a brief summary of my talk here.