The latest RCSB PDB release features a first version of an iPhone application. It is provided as a HTML5-based application, which means you can install it without going to the Apple Store. Simply point your iPhone Safari-browser to http://www.pdb.org and click "yes" a couple of times. Best to do this while you are on a wireless connection, since the application installs some data for quicker data access.
Gregg, the author of this application also made a screencast with the installation instructions. You can watch it here: