Hi,
there's a Plugin for PS (Windows only) called Curvemeister which allows to Curve in different colorspaces.
E.g. the document is RGB, the Curves dialog shows the usual RGB Composite + R, G, B channels and L, a, b, and C, M, Y, K (which channels are calculated on the fly).
In PrePress it's quite usual to switch carelessly from one colorspace to another to exploit each colorspace peculiar features. The multi-colorspace Curving could be implemented, say, only when the document is converted to Lab (so that you're already in a device-independend space, which saves you a conversion).
