I'm no expert in this subject, but mxyzplk has written a good summary of the issues affecting open gaming and the upcoming release of 4th Edition D&D. The open licensing associated with the 3rd Edition spawned a number of successful 3rd parties and add-ons that made the system far greater than it might have been otherwise. I've attached his writeup on the subject below, and you should really read it if you are interested in D&D, Gaming, or trying to apply 'Open' licenses to things besides code.