All lettuce contains the soporific drug. They have tested the same drug on humans and 'apparently' (according to Metro a few days ago) in large doses it causes a viagra effect, but in small doses it causes a soporific effect. The main problem with Iceberg is the water content.
I occassionally feed mine round lettuce, romaine lettuce and little gem, but I wouldn't feed it to any rabbit unless he/she is used to having lots of greens because it can cause runny poo. If you're unsure and your vet says no, I would say just avoid it to be safe.
Having said that, my own two have never appeared to have any problems with eating small amounts of these types of lettuce.