I have it on good authority that the plastic used in the Acerbis tanks can be 'field repaired' as well. apparently it is similar to that used in plastic jerry cans, and other common containers. New material can be melted in to repair cracks and the like.
I have tried this with other objects, and it is possible given enough patience.
On another note: a friend of mine modified a steel tank from a different bike to create greater fuel capacity. dispite careful welding, we spent a lot of the trip repairing hairline cracks that developed where the tank had been modified to fit. Metal can be a pain in the butt. Having seen the effects of vibration on aluminium boats, I am not sure I would want an aluminium tank on any off-road going bike!
Mind you, they do look nice all polished up....