Zaralynda has a fantastic answer, that runs counter to my in belief the "badness" of iceberg lettuce for bunnies. In an attempt to support my beliefs I did lots of research, the following is what I learned.
We all agree that Iceberg Lettuce is not a good source of nutrition, but is it actually bad for rabbits?
There are several mentions online and in print of iceberg lettuce, containing laudanum and being related to Opium producing plant Papaver somniferum, in fact iceberg is no closer to this than any other lettuce Lactuca sativa. Plants are scientifically classified in the Kingdom Plantae (this is the division between plants and animals). The nearest classification they have in common is Eudicots which includes many flowering plants including apples and maple trees. This pretty much destroys the relationship myth.
So where did the myth come from? It seems that many plants including lettuce have a bitter latex like substance that has been used for a substitute in producing the soporific effects of opium Flora of Great Britain and Ireland, Volume 4 By Peter Sell, Gina Murrell, page 117. Lettuce appears to have been a popular source since the time of the anciet Egyptian, this was generally collected when the plant had bolted (flowered). The drug is called Lactucarium, it does not appear to hold it's effects well in commercial productions, but does appear to be less disruptive to the digestive system then opium.
I did not find reliable references on the bloat and gas concerns related to iceberg lettuce
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