I don't know if this feature is known, but I've just discovered it.
Highlight a layer in the Layers palette. Double-click with the Crop Area tool
on the artboard. The crop area is sized to encompass the objects in the
highlighted layer. Highlight a different layer, double-click with the Crop
Area tool and the crop area re-sizes to fit the objects in this layer.
Highlight multiple layers, double-click with the Crop Area tool and again the
crop area re-sizes to fit all the objects in these layers. Nifty.
--
Michael
mhphillips at gmail dot com