I recently found a great set of stencils for wireframing iPhone apps and mobile websites in Visio. The stencil set was created by Jonathan Abbett of Beacon 16 and is available under a Creative Commons License.
The stencil shapes include:
- Navigation Bar
- Search Bar
- Browser Bar
- Four and Five Button Tab Bars
- Date Picker
If you need a higher fidelity GUI design toolkit for iPhone and iPad, Speckyboy Design Magazine published a great list of resources last year. If you need to illustrate touch gestures in your wireframes, Luke Wroblewski maintains a Touch Gesture Reference Guide on his website.