Every year about 50,000 people travel to Gansbaai for a close encounter with the area's great whites, drawn in by the presence of a huge colony of Cape fur seals, and each day boats set out to sea to give tourists a closer look. Dangling bags of "chum" - usually mashed fish - a scent trail is created, and pieces of tuna on a line are used to draw the sharks towards divers in a cage on the side of the boat. (http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2008/02/09/wsharks109.xml)
The article's focus is that there is fear among locals that the chumming to attract the great whites could cause the animals to start associating humans with their natural food (tuna in this case). As far as I know there has not been any scientific evidence that chumming might influence the sharks to associate humans with a food choice. However, it is my opinion that it is truly a sad reflection on our society we would play this dangerous game with an endangered species.
Lets say for example that there is some truth in the idea that the chumming could condition the great whites to start associating humans with food. And lets say that humans (as is claimed by most experts) do not "taste" well to great whites (in other words, great whites do not like how humans taste). The problem is still that sharks will take bites out of humans to be sure that nothing has changed.
I don't know if it is fair to make this comparison, but as most know, there is a bit of a parallel with the behavior of the apex predator of the bush - the lion. Lions do not typically see humans as part of their food, but once a lion has crossed the line and attacked and perhaps killed a human they then learn a new behavior that humans are a food source. These lions are most often killed because of their "man hunting" behavior.
I know that I am speculating a lot in this post. The hypothetical question that I am posing is what if cage diving and chumming causes some great whites to change their behavior and start attacking humans on a larger scale? I think that the natural reaction from humans will probably be to hunt the great whites out of existence.
I have done my share of diving with sharks (not great whites). It is my opinion that it is best to observe marine animals in their natural environment. If that means waiting a little longer for an awesome photograph then so be it.