Try this if you're using OS X 10.4 and Bonjour...

1. Launch System Preferences in your Applications folder
2. Choose the Sharing icon under Internet & Network
3. Select the Firewall tab
4. Click New

Enter this information:
Port Name: Other
TCP Port Number(s): 60000-62000
Description: OmniFocus Bonjour

Now try the normal steps to sync your iPhone.


Note: I'm not entirely sure what the port range it. It seems that the phone attempts to connect to ports in the 60,000 range till it finds one that's open. That makes it difficult to configure your firewall properly (and securely).

The Mac OS 10.4 firewall works by denying everything by default then allowing only specific incoming connections based on ports.

The OS 10.5 firewall, by default, allows all connections but restricts which applications can open ports. This is probably why 10.5 has fewer firewall issues than 10.4. Convenience at the cost of security.