I'm sure the SQL backend can be clustered - we currently have our 4.5 db on a MS SQL cluster. But I doubt you would put the Service Desk Server application on a cluter - you shouldn't need to because you can have multiple application servers anyway so clustering is not needed?
Thanks for the SQL information, but what I really need to know is if the application server itself can be clustered as we are looking for our cluster fail-over capabilities to also include hp openview service desk.
just a theory as I didn't have an opportunity to try it - you can create an IP address as a cluster resource and let your users use this IP to connect to your SD server. Then, make sure you'll have "Join multiple servers" option selected in SD "Server Configuration" serving as a load balance and as a failover.