MC ServiceGuard is still at the forefront of HP-UX's high availability software, it has been developed to the point where (providing hardware behaves itself) it can provide 99.999% availability when spreading a service across two service guarded machines.
It's quite a sophisticated product, providing a double heartbeat between nodes to monitor machine availability in a cluster and has a whole host of cluster wide and node specific configurable parameters - HP's course covers 5 days. It works from the entry level A class server through to the top end Superdome, as well as on workstations (although the reasoning behind it's application there would be somewhat baffling).
The product does not pretend to create fault tolerant machines, I think fault tolerance in the strict sense of the word is something yet to be attained on servers, and the domain of the mainframe - but it does guarantee high availibilty and with most service level agreements providing for scheduled maintenence downtime a high availability cluster that is service guarded can provide a near fault tolerant environment by allowing services to be flipped from node to node as the sectrions of the cluster is worked on for maintenance.
It's a great product, I would recommend learning it. I am on the course next week
Regards.