There are no useful examples for setting the property of this type anywhere. The GeoShape page references the rNews GeoCoordinate spec in passing and a forum post on meetup states that this spec is based on Simple GeoRSS.
I have guessed that the Simple GeoRSS coordinate format is correct. Unfortunately, Google's Rich Snippet Testing Tool doesn't inspire confidence in that conclusion.
This is the ideal data format for my purpose (describing a service area of 20 miles centered on the city centre) so it is really frustrating that I can't test the correctness.
This is a duplicate of my question on StackOverflow.