There are two types of certification authorities: Standalone and Enterprise. Only Enterprise certification authorities have been tested for clustered installations.
A very short but may be important statement.
Can you possibly elaborate on whether multi-site clustering is supported? My plan is to use multi-site clustering on a subordinate CA server for near-instant failover/recovery when using cross-site sychronised SAN volumes.
This question is not CA related. It's more a question what the underlaying cluster is supporting. See http://download.microsoft.com/download/8/C/2/8C21BAFE-3432-48D1-962A-F7A9DD54A2AC/Multi-Site%20Clustering%20with%20Windows%20Server%202008%20Enterprise.pptx for related information.