As was pointed out to me just in another question the 4th ed / D20 licence switch was not a revoke of the D20 open license. Although what I remember from that time is that publishers were forced to decide to either continue publishing D20 OR switch to 4th and all new publishers could only use the 4th edition to publish and not the D20 one.
As I said it was pointed out to me that my interpretation there is erronous so my question is, is there any info on what really happened there?