Disagreement over definition of l,b coordinates

DS9 does something intuitive: cd0_0 and cd1_1 multiplied by the x,y pixel difference gets you the new l,b coordinate. I think this is very likely to be correct on the galactic plane. IDL does something different. I don't know why different, or really how - it's buried somewhere in wcsxy2sph - but it's definitely different. Which is right?