This is really dependent on the NAS that you are utilizing but for the most part, you just define multiple Protection Engine Servers on the NAS. For example, with EMC, you would put multiple ICAP servers that are the SPE servers. If one is down, the Esilon will fail over to the next. Netapp is a little different, as it utilizes RPC.
Another option is to use a VIP (Virtual IP) on something like a F5 Big-IP for HA. The preferred method would be to configure multiple SPE servers on the NAS.
Hope this helps,
David Ramazetti
Senior Engineer
Novacoast