mapRenderer = iface.mapCanvas().mapRenderer()
llx=729200
lly=4361200
urx=736400
ury=4368400
rval=mapRenderer.setExtent(QgsRectangle(llx,lly,urx,ury))
print "  lower left = "+str(llx)+","+str(lly)
print "  upper right = "+str(urx)+","+str(ury)
print "rval:"+str(rval)
print "mapRenderer.extent():"
e= mapRenderer.extent()
print e.toString()

>>>execfile(u'C:/Users/Tom/Documents/extent_test.py'.encode('mbcs'))

  lower left = 729200,4361200
  upper right = 736400,4368400
rval:True
mapRenderer.extent():
727776.0840998685453087,4361200.0000000000000000 : 737823.9159001314546913,4368400.0000000000000000