Yes, I tried Kentico on SQL cluster and mirror as well. As mentioned, it is just the SQL server thing. It depends what you mean by Active-Active - if you mean web servers, like load balancer or web farms, then you have to buy additional license for Kentico. If you mean the SQL server - this should be answered by Microsoft :-)
Disk array - again, this has nothing to do with Kentico. You should ask this some hardware or performance specialist. It depends on your project, its traffic, what availability you want to have and many other things. Small web site can run on a laptop, big web site needs load balancers and failover servers - the range where Kentico can be used is very wide.