Yep, Mine's aluminum. It's 60mm OD and about 5 or 6mm wall thickness so it's plenty strong.
I think most of the steel or stainless ones are about 2.5 to 3mm wall.
According to CAMS the 3 point single hoop only compliant for speed events (Sprints etc) with no passengers. For races the minimum is a 1/2 cage (4 point with a diagonal cross brace).
Tubing needs to be steel and 1 5/8" x 2.5mm wall or 2" x 2mm wall.
I plan to make a steel half cage for mine for track days. I'd like to be able to take a passenger for a spin. The alloy bar is for the road.
There's some good info here:
http://www.camsmanual.com.au/pdf/10_...ule_J_Q109.pdf
Cheers