[Joel]

You're going to need to get the OmniFocus 1.1 sneaky peek, which is the release that includes syncing to WebDAV.

PLEASE ENSURE THAT YOU BACK UP YOUR EXISTING OMNIFOCUS DATABASE BEFORE UPGRADING TO VERSION 1.1.

Your existing OmniFocus database should be located at ~/Library/Application Support/OmniFocus/OmniFocus.ofocus. Just squirrel a copy of that away somewhere before launching the OmniFocus 1.1 sneaky peek, as the new version will upgrade your database into a new file format.