I've been using OmniGraffle Pro for most of my iPhone and iPad wireframe jobs. I love the product, simple and easy to use. One of the things that I dig the most about it is the vibrant stencil and template community at http://graffletopia.com/. There are a number of stencils and templates that you can find out on the internet. The two that I've found to be most useful over the past few months are:
They are well thought out and I've been able to find and modify just about any component I've needed.