Amazon already has technology to remotely delete books from Kindle. In fact, it has already been used: http://www.nytimes.com/2009/07/18/technology/companies/18amazon.html
Thanks a lot folks for your opinions. After reading your posts and thinking carefully to myself, I cleared my mind: my problem is not paper vs. ipad/kindle per se. My problem is more about how to keep track of the vast amount of trading info and search later. So I am going to create a new thread to discuss the information collection/maintenance in the digital age... Thank you!