Iceberg lettuce is bad for them they can eat the darker varieties like romaine but I tend to only give them a bit of the stalk if I am having some I don't use lettuce as a staple.
Cabbage is high in calcium so is bad for bunnies with calcium problems but I have never known it to kill. Mine get cabbage all the time (green, white and red) and they love it. Brendan has even given them whole to the pairs before (when I said to give them cabbage I should have been more specific on portion size) and they had half a cabbage each devoured in one night with no ill effects!
That is the first mention I have heard of it being dangerous - did they have any evidence to back this up or a reason for stating this?
Edit: Oops fast fingers Adele! Mine sometimes stink after eating it (urine and poo reeks). I have cut back as they can get a bit windy and with Zeus on pencillin I have to be careful of his guts but even with Zeus and his delicate digestion I have had no problems with cabbage to suggest it can kill if the bunny is used to eating it.
Caz