Most of the Web UI and iPhone UI people are using Yahoo Design Stencil Kit for making UI mocks. Yahoo provides it in various formats for use in various applications like firefox, svg, etc.

We just need to repackage this to use it in Pencil.

I have created a quick and dirty stencil set using the stencil generator with Yahoo Design Stencil Kit. And it is usable . . . I think one made from the SVG will be better. Will try that when I have more time at hand. Please try out. Will improve it whenever I get a chance.