Please note that this issue is resolved. If you requested a Maya 2012 network license after 25th April and you have a valid Subscription contract, your license will include all the necessary activation codes to run Maya 2012, 2011 and 2010.You will not have to resort to copy/pasting parts of your Maya 2011 license to activate the fluid simulation and mental ray render nodes in Maya 2011.
Your Maya 2012 no longer requires the fluid simulation and mental ray features (including render nodes) to be activated in separate codes. All features of Maya 2012 can be activated from a single activation code. However, a correct license file - issued after the 25th April - will also include PACKAGE and INCREMENT lines explicitely for those features in Maya 2011 and Maya 2010 that require separate activation codes.
If, after you start using your Maya 2012 network license you find that you can no longer render using mental ray or use fluid simulations in previous versions of the application, plesae read the rest of this blog post..