To compute distances or areas, use QgsDistanceArea class. If projections are turned off, calculations will be planar, otherwise they’ll be done on ellipsoid. When ellipsoid is not set explicitly it uses WGS84 parameters for calculations.
d = QgsDistanceArea()
d.setProjectionsEnabled(True)
print "distance in meters: ", d.measureLine(QgsPoint(10,10),QgsPoint(11,11))
TODO: area, planar vs. ellipsoid