Q. “When am I supposed to report SQL core licenses?”
A. When you sign a new SPLA or you deploy SQL 2012 you will be forced to license by cores
Q. “If we leverage SQL Server Enterprise, are we able to launch multiple VM instances of database across the enterprise?”
A. SQL 2012 Enterprise allows unlimited virtual instances. In order to this with the 2008 version you would of had to license Datacenter edition. (way expensive btw). SQL 2012 does allow license mobility within server farms as well. Check out the SPUR for details.
Q. “Does SQL 2012 Enterprise edition allow for downgrade rights? In other words, can I have 2008 SQL servers and 2012 SQL servers running virtualized on the same host?”
A. Yes, as long as you are reporting SQL 2012 cores, you can run 2008 and prior. Keep in mind, it has to match version. For example, if you license Standard, you cannot run Enterprise. If you license Enterprise, you can run Enterprise or Standard.
Q. Can I just license the virtual, not the physical machine?
Yes. SQL does allow you to license just the virtual machines. You would report the number of cores you assign to the server. (minimum of 4 cores).
Q. Can I license SQL Enterprise by user?
A. No. Unfortunately SQL Enterprise can only be licensed by core. SQL Standard and SQL Business Intelligence SKU can be licensed by user.