[Andrew]

All of the communications at that level is dealt with by the phone. I'm afraid I can't think of any way to diagnose it.

The one thing that I can think of is that maybe you're using HTTPS for syncing? (This would be the case for iDisk syncing, or WebDAV if you've specified HTTPS.) If so, make sure that you can also do HTTPS in your web browser over 3G, by going to a secure web site. (I expect you to be able to do HTTPS fine, it's just the only thing I could think of that might be different in OmniFocus from "normal" web browsing.)