As part of the research into why grapes and raisins can be so toxic to dogs' renal function, researchers are looking into the possibility that grapes might contain mycotoxins. I can't find any reference to their findings so assume the jury is still out.
Mycotoxins are of course potentially fatal to rabbits. I haven't fed grapes or raisins to my buns for many years and would like to see the result of the mycotoxin theory before introducing them. :|