Krueger: Yay. My area of expertise.
Man was going to age by time, and he'd mature physically, but he would not approach death by doing so. Presently, aging leads to death because the body's cells have a limited number of divisions, so as you grow and your cells divide, or as you get injured and cells reproduce to repair the damage, you use up the number of divisions the cells have left. the inabilty to continue to divide causes a lot of things, like wrinkles in old skin, and death by natural causes. It is even stated in the Bible that once we are born we begin to die. Mildly depressing, eh?
Anyway, the generally accepted idea as to the purpose of the tree is free will. God gave man (and later woman) paradise on the grounds that they would follow his one command and basically live in a blissful stat ignorant of the faults of their actions. If there were no tree, man would practically be forced to forever remain in this ignorant state. By offering the tree, man had a choice: he could continue on in his present way of life, or he could live a life full of sacrifice and pain BUT he would no longer be ignorant (plus, he would momentarily have the pleasure of eating the fruit). God obviously wanted the best for his creatures and so commanded the avoidance of the fruit, but Satan had other plans. Just because something happens doesn't mean it is something God likes. Therefore not all catastrophe is his desire. I suppose he knew what would happen, but he had the plan to send His Son worked out already anyway, and I suppose he just didn't like the idea of a forced obedience more than he disliked the idea of a chosen disobedience. After all, if it's forced, is there really devotion? By having the ability to choose disobedience, it requires devotion now to be obedient.
The reason the God cast Adam and Eve from the Garden was not so that they would die. They were going to die once they touched the fruit. He kicked them out specifically so that they would not change their fate and live forever. Adam and Eve had just gained a very godlike quality with their newly acquired knowledge. Now if they lived forever, they might become even more likely to turn to corruption in their state of elevated power. Also, they were told they were going to die and death would be their punishment. To uphold both God's truth and authority, this had to remain true, and so the one thing that could prevent their death, eating of the tree of life, had to be prevented.
Also, Adam and Eve may not have known about good and evil before eating the fruit, but they were commanded against it AND warned of consequences beforehand. It would be these things that they would use to convince themselves (or not, sadly) to stay away from the tree.
Woo. long post. Hope it was helpful...if you read it all.