OmniFocus for iPhone was updated today to include a background location sync feature previously seen in apps like Downcast, Instapaper, and CameraSync.
Users can now leverage iOS' geofencing to tell OmniFocus to sync its online database in the background and automatically whenever they enter or leave a specific location. There's a new Background Sync screen in the settings where locations can be configured, which works similarly to Instapaper's one. Unlike CameraSync, I wasn't able to receive a local notification when I left or arrived at a location that triggered background sync.
The Omni Group is undoubtedly looking at iOS 7's new background app refresh options, but, until then, this is a nice (and common) workaround.