Open sourcing (making free) your content management system has nothing to do with your ability to sell it. It may even help. Your clients could still hire you to extend it. It is not a matter of being evil or legally protected.

Of course I'm curious about your system too, but you have to make sure it won't damage your income. I think it would help really, since you could get more jobs, and requests for customization, which you might not get otherwise. You might also get help with localizing it or adding functionality. This could happen more easily if it is made free.

As for legal questions with your clients, you might want to put in your contracts that it is a site liscense, they can't redistribute, and that you are not responsible if they modify the system. That way you get feedback and work, and less hassle.