Wizards of the Coast, a subsidiary of Hasbro, is releasing a new license with the upcoming fourth edition of Dungeons of Dragons. Publishers can create compatible D&D products, but only if the companies no longer publish any games which are distributed via the Open Gaming License.
This would be like saying that developers could not run programs on Vista if they publish -any- programs under a GNU license. Keeping up with the D&D 4th edition "GSL" license situation might be important, because it could very well be a precedent.
Update: The license itself has not been released, but the linked article below contains links to message-board postings from senior Wizards of the Coast employees that seem to validate this view of the license.