I have an EdgeFalloff shader that should be what Paul is after.   Its good for cheap special effects like soft edges on clouds, smoke and for transparency on glass.

I cut and paste a few lines of code from my MipMapper plugin into a new project quite a while back that gives this 'EdgeFalloff' effect.  ( It takes the dot product of the surface normal and the vector from the camera - simple, once you get hold of the camera ! )

Most other applications have it and it really should have been included with Carrara in the functions list many versions ago...

I was going to bundle this up with a few other basic shaders but if anyone wants to try it now...sorry, still only  Windows versions though.

http://sparrowhawke3d.com/Downloads/EdgeFalloff/EdgeFalloffC4.zip
http://sparrowhawke3d.com/Downloads/EdgeFalloff/EdgeFalloffC5.zip
http://sparrowhawke3d.com/Downloads/EdgeFalloff/EdgeFalloffC6.zip