Searching for OpenPGP in the App Store returns two apps only: SecuMail and OpenGP. Both interoperates with GnuPG. SecuMail is more robust in terms of functionality but costs a whopping $49.99. OpenGP (the Full version) is priced at $7.99 only, but it currently operates in a copy-and-paste message only — no support for attachments.
There are a lot of message encryption apps in the App Store in various encryption algorithms from 64 to 254 bits and in various prices. Some are universal apps for both iPhone and iPad. But most if not all are offering symmetric encryption, meaning you have to provide the other party you send your message to the passphrase in order for him/her to decrypt the message.
So, if you are already using OpenPGP like GnuPG to sign and encrypt emails in your desktop computers, you would love the two apps I mentioned above to access the same signed/encrypted email messages from your iPhone.
Update (June 3, 2011): There’s a new app that’s much cheaper to both apps featured above. It’s called iPGMail. The app costs US$ 1.99, and it appears to be functioning very well based on users’ reviews in the App Store. Wish I could try that to test, though.