There's a bug in the latest release with the initial sync. Here's the workaround:
I'm having trouble setting up Synchronization in OmniFocus for iPhone 1.5 under iPhone OS 3.0. Can you help?
There's currently a bug in OmniFocus for iPhone that may prevent you from successfully entering your credentials when syncing. To get around it when setting up synchronization:
On the first launch screen, choose "Make New Database" (counter-intuitive, right?)
Go to settings and set up sync as normal.
Go back to the home screen, and if sync is spinning without prompting for a username and password, quit OmniFocus.
Relaunch OmniFocus, press sync and you should be prompted for a username/password.