> Is all this intended behaviour?

At the moment it is, but I had been thinking of changing this to use the plane of the object instead. I'll see if I can set this up for the next beta release.


> What would i have to do to e.g. offset a circle that is outside of a system plane
> so that the offset curve is coplanar with the original?

Let's see - right now I guess you have to rotate it back to a world plane, or draw a new version on a world plane first and then rotate it back into place.

Although here is a different method that is a bit convoluted but I think is actually easier than rotation - select your circle and extrude it by a little bit, it doesn't really matter how much. Unlike offset, extrude does use the plane of the object. Now you've got a cylinder - select the side face of the cylinder and offset that by your desired amount. Now select the bottom edge of the cylinder, do a Ctrl+C to copy it to the clipboard. Now undo the surface offset and undo the extrude, and paste the bottom edge in, which is the offset circle that you're looking for.

- Michael