[Lizard]

1) On iPod Touch, in Settings, tap the big red button. ("Reset Database")
2) On Mac, quit OmniFocus. Open Library/Application Support/OmniFocus in your home folder and delete (or rename) the file named "OmniFocus.ofocus"
3) On your WebDAV server or MobileMe, find the OmniFocus.ofocus file and delete (or rename it).

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That should clean out all your existing data. Put a test task or two in on the Mac and make sure it syncs to your iPod before adding more stuff.