Passive... is a very relative term But, as far as SQL server licencing is concerned if you are 'using' it even readonly or DBA activity then this is deemed a 'Production server'. I know this is also an over used term but the only 'passive server' that doesn't need a licence applied is a mirrored server in the pure sense of the word. The area that is hitting the DBA at the moment is the understanding what Mircosoft refer to as 'Data mobility'. There is not one answer. Trust me, we've been tooing and froing with our licence provider over this for a long time. Their original bill was, to say the least, un-believable and 6 week later we have managed to cut it by two thirds, the CIO is happy. There is no one answer and the calulation is convoluted for instance.
Case 1. (I'm using rough figures $10K per STD, $30K per Enterprise, datacenter I'm not even going there.....)
1 SQL Standard installation on a dual quad core server = 1 licence per Physical CPU (black plastic on the motherboard) hence $10K X 2 = $20K.
The same SQ server install on a Virtual server using the cores per cpu division rule, need only 1 licence, working like this
1 SQL std using 2 Virtual CPU's (on a dual quad core host) 2 / 4 is less than 1 there only 1 licence. As common sense would dictate in that in a virtual environment you don't get to use all the horse-power of the CPU there MS uses that number of Vcpu's/the number of cores to determine the license count required. Therefore only $10K .... Ahhh I hear you say well we'll just put all the SQL servers on one host, give them 4 VCPU's "huzzar" I hear you say, 'supedup SQL servers', and to a point that can work. But remember you can only fit 5 Lbs of excretment in a 5 Lb bag and if start over subscribing the Host then you are in a world of hurt.
I hear the ESX/Hyper V boys and girls going "but we have dynamic resource allocation and clustered ESX hosts with 24 cores and 196Gig of RAM, not a problem." -- Wrong !! Because if left to the defaults and you let you virtual environment management move the "SQL Server" or "OSE (operating system environments)" as MS describes between host and because the STD edition doesn't have this licence mobility, the calculation is very VERY different.
1 STD SQL Server with 2 vCPU's with 3 possible hosts equals 2/4 there 1 license x 3 possible locations by $10K = $30K . Now do the same calulation with Enterprise Edition(licensing the vCPU's) 1 Ent using 2/4 = 1 licence with 3 possible hosts = 1 x $30k x 3 = $90K.
Don't just take my word for it check this out http://tonymackelworth.wordpress.com/category/sql-server/
This is not a new thing as this licensing model was introduced in 2008.
The break even point is when you calculate the cost of an Enterprise license per physical CPU model eg 6 Physical CPU ESX host where each is licenced starts looking competitive 6 x $30K = $180 K with each licence you can install 4 OSEs, up to enterprise edition, therefore 24 STD editions if licenesed individually would be 24 x $10K = $240K. In our environment this worked, yours will be different.
There are so many variations that each environment needs to be investigated taking into account the virtual hosts, High availability options used, a whole raft of things.
BTW the new version of ESX (4.0) has migration rules that can be utililised to prevent migration between host groups and there by possibly reducing your TCO of SQL server.
Have fun. This is the brave new world-Virtually.