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The postGIS GEOGRAPHY type connects two points on the Earth’s surface by the shortest route (the great-circle line). Unfortunately when displayed in Quantum GIS, this will appear as the shortest route between two points in map-space (actually a rhumb line).

To display a great circle line I have used the following postgreSQL code:
SELECT st_makeline(st_segmentize((SELECT geog FROM paths WHERE gid = 1), 50000)::GEOMETRY);

This returns a GEOMETRY line string, with a node every 50 km. Useful if you want to plot idealised flight paths…