You can run an applescript which picks up the longitude and latitude of the current view in Google Earth, and applies them to the context(s) currently selected in OmniFocus.

http://forums.omnigroup.com/showthread.php?t=19072

You can also view the location of a selected OF context in Google Earth.

(A way of checking that you have set the right latitude and longitude. The view will be centred on coordinates specified in the context's location.)

http://forums.omnigroup.com/showthread.php?t=19078

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