Game systems are open by default unless they are patented. In Wizards of the Coast’s case, they specifically designated their game system as an open gaming license with their third edition of Dungeons and Dragons. This was a good move, as thousands of products labeled themselves after the idea, boosting the D&D system in turn.
Apparently the party is over, as Wizards plans on telling all other publishers: if you publish anything that uses the new D&D system, you can no longer publish anything under the previous open gaming license. So you have to choose: support D&D 4the edition, or support open gaming.
I am not a lawyer, but I don’t see how this can be enforced, since systems are not protected unless they are patented (copyrights are not enough, as they only protect the wording, terms, and pictures). But Wizards can probably make your life difficult enough that, even if they would not win an eventual court case, you’ll probably go broke in the meantime.
Update: Wizard forum posts seem to belie this interpretation, as noted by Boing Boing a week or so later (May 3). (source)