I have always been a fan of a simple sketch rather than any of the huge array of available prototyping tools. UXPin might end up being the middle ground between the two by using paper and sticky notes to lay out iOS UI elements which are emailed, recognised and then converted to an editable prototype.
An alternative to above if you prefer the high fidelity look for your mockups.
Jiva DeVoe notices that tapping just to the left of the 6 glyph on the iPad split keyboard will actually give you a 5, super cool. I can't think of any other platform (even Mac OS X) where this much care has been taken with user interaction.