Omnifocus for iPhone
1. Tap settings gear in the bottom righthand corner of the main screen.
2. Tap the 'Sync Method' field.
3. Tap 'Custom (WebDAV)'.
4. Tap the Address field, and enter "https://dav.dropdav.com/" followed by the path to the folder you want to store the OmniFocus database. Don't forget a trailing slash on the end!
5. When you're done, tap 'Sync' in the upper righthand corner.
6. Enter your DropDAV email address and password.
7. Tap 'OK'. You're all set!