Hi Tony, some previous discusssion on unwrapping cones here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6175.1

I think there is a plugin script (or maybe 2 of them) in that thread as well.

You can do it with some construction geometry as well I think . Get a line on the other side of your cone and then select that line and the edge of the cone to do a "mutual extend" so they meet at the apex point:





Then select that line and use Transform > Rotate > Rotate axis to rotate it so it's on the x/y plane, and that then becomes the radius of the outer circle on the unwrapped shape.

- Michael