I'm looking in to some upgrade costs for a few systems and was told by our happy Oracle rep. that full Oracle EE licenses are required for all standby databases (i.e. physical standby data guard). Costly indeed.
Now I stumbled across this PDF on the Oracle website.
From the PDF:
"System: shall be defined as each distinct production database. Test, development, failover and standby databases are not required to be licensed as systems; however, you may run the program on these databases."
Not sure whom to believe.