I think it would be a great idea. I also think it should be setup so that you can not "out gear it". It should strip you (like Challenge Modes) so that it is more reliant on skill. And having the reward do a significant spell visual change so that people know what you have accomplished (like green fire). I think it would be totally awesome.

In addition, they could add new ones each expansion that are even harder than previous ones. Good stuff. I think a lot of people would enjoy the challenge.