Will never happen. In the history of games, very few companies have ever released their proprietary Intellectual Property to open source. They just moth ball it.
Also, if they released the game code it exposes the game to potential hacking. Part of what keeps WoW safe (well safer anyways) is that hackers do not have access to the backend code to analyze.
Finally, Blizzard will not do this because it would potentially reduce the overall quality of WoW. Open Source would mean anyone could make their own modified version with no guarantee of stability or security.
It is the same reason why Microsoft and Apple do not open source their software.