Is it supported to install a Certification Authority service on a Windows Server 2008 R2 joined on a 2003 native functional level domain?
Is it supported to install a Certification Authority service on a Windows Server 2008 R2 joined on a 2003 native functional level domain?
Yes.
However, to get the full functionality of the newer CA (particularly version 3 templates) you'll need to run the 2008 schema upgrades (I don't believe the R2 schema changes impact certificates at all) - which does not require a functional level change or even any newer domain controllers.