For those of you who’ve never used it, Omnigraffle is a wonderful visual design tool that can be turned to all sorts of tasks. It can create any manner of diagram, but works even better when enhanced with template themes that add specific visual widgets.
This particular set of widgets gives you almost everything you’d need to mockup an iPad app of your own. It includes drop-downs, alerts, the software keyboard, and loads more. Various bits of text are customizable, so your mockup looks as real as possible.