The people who make Foreflight for the Mac (and peecee) now have an iPhone version.
It's very well done with a UI that is appropriate for the phone. In addition to weather (including radar images), it provides airport information (diagrams, AF/D data, etc.), sectional charts (integrated with SkyVector), N number searches and other features.
Flight plan filing (with DUAT) directly from the iPhone is in public beta now.
They've recently added automatic integration with LogTen Pro logbook software. It lets you view currency expiration dates, flight totals, etc. on the iPhone.
You can get more information here:
http://www.foreflight.com/
Disclaimer: I'm not affiliated with the company in any way except as a customer.