Johnny Long, of Google Hacking and other fame, posted a quick article on Wednesday, 01 November 2006 titled, "How can I become a professional hacker?".
It is a great article and everybody who has this question should read it.
As related to Ethical Hacking, Johnny says:
If you fall victim to the inevitable desire to hack a server that doesn't belong to you, and without permission, give up. You'll get arrested eventually, and your trust will suffer an almost certain deadly blow. If you're not in the game to be one of the good guys, give up. Your ultimate goal should be to protect your clients.
Go forth and do good things,