We do need this behaviour updated so that if you have a scene saved in 1.5 some kind of "legacy" checkbox in RoundedCorners will be enabled, so that images render the same. We've just re-rendered some 1.5 scenes in 1.6 and have all sorts of issues with this. Yes the geometry shouldn't in an ideal world be modeled this way but as someone said earlier, when the design is changing almost daily sometimes it's best to keep this sort of overlapping faces for flexibility in re-modeling....