Sorry you have the information incorrect. Cyan is going to allow access and to share some of the source code but not total open source. Fortunately they will oversee and keep the rescources protected from wild and chaotic exploitation, something a small minority of the game community for URU have tried to do for years. The URU story is a wonderful one and one of the best Multiplayer games developed yet for there is no killing, no violence, no greiefing, no faction boring resource grinding, and a wonderful and deep story line and beautifully created 3D world. It is really a work of art. This move by Cyan will allow the very large international community to return to the game world and for the chance to add some new content that will be regulated internally by the Guilds that are part of the game world. This is NOT open season free for all or a model for such for other games. Any mod work should reflect and continue the intent of the original game and honor the effort put into it. Most modders know this and honor it.
You can find a good summary of the first year of MOUL Myst Online URU Live here: