BATTLE MAP (for iPad) Compared to all the other apps here, this is insanely expensive at thirty times the price of most. Create and explore your own pen & paper RPG maps with Battle Map - as simply as dragging your finger. Battle Map will automatically beautify tile edges, calculate line of sight, and reveal traps - on your iPad or an external display! $29.99
RPG CARTOGRAPHER (for iPad) Another vitual tabletop mapper. Similar to BATTLE MAP but a third of the price, though not quite as fully featured. $10.99. Demo video here.
POCKET TABLETOP RPG HELPER Not much more to it, from the looks of things, than a simple and not too pretty dice roller. Free. The makers of i4E have a new iPad-optimized app for browsing the D&D Compendium:
Compendium for iPad on the iTunes App Store
It's free but only for the iPad at the moment; I'm hoping they do an iPhone version at some point.