Found this on a google group:
Re: License For Replicated Windows Servers
Microsoft offers what they call “disaster recovery rights” for every license that you have covered by software assurance.
“Cold” Disaster Recovery Rights.
For each instance of eligible server software you run in a physical or virtual operating system environment on a licensed server, you may temporarily run a backup instance in a physical or virtual operating system environment on a server dedicated to disaster recovery. The product use rights for the software and the following limitations apply to your use of software on a disaster recovery server.
The server must be turned off except for :smileyinfo: limited software self-testing and patch management, or (ii) disaster recovery.
The server may not be in the same cluster as the production server.
You may run the backup and production instances at the same time only while recovering the production instance from a disaster.
Your right to run the backup instances ends when your Software Assurance coverage ends.
As part of its Software Assurance licensing program, Microsoft currently offers free software licenses for cold servers intended for disaster recovery. It does not offer free licenses for warm or hot servers. You get more details in SRM Reference Guide.