Steve has some good points in this article but I disagree with his recommendation of quitting your current job. Not having the piece of mind from a steady paycheck coupled with trying to decide what to do with your life can leave you broke and in debt.
A better idea is to: 1) make a plan for change 2) analyze other careers 3) interview people in careers that your are interested in 4) Decide to change 5) make plans to change 6) reduce/eliminate debt 7) Find a new job in your new career 8) When found, then quit!
I can't stress the importance of aligning your finances when wanting to change. You will take a pay cut.
I posted this link in another thread and feel it's relevant to this topic:
CoolPeopleCare | Making Your Passion Your Profession
I speak from experience on career changing. I have not made the leap, yet. However I've been prepping for the past 3 years. Everything is aligned and I'm waiting on new opportunity. It's frustrating and I fight the urge to quit everyday. However I know I'll be better off by being prepared.