Plan high availability

Service redundancy

More on Microsoft failover clustering here

Microsoft failover clustering no longer supports direct parallel SCSI and requires shared storage that is SCSI Primary Commands (SPC-3) compatible i.e. uses SCSI-3 persistent reservations.

Service availability

More on Microsoft Load balancing and DNS round robin here

Mixed-mode clusters are possible but it is recommended that all cluster  nodes are running the same operating system.

If IGMP multicast mode is used then IGMP snooping must be enabled on the switch.

Each cluster host sends a heartbeat every second, if five consecutive heartbeats are missed then the cluster converges removing the failed host.

The MCITP self-paced training kit for Windows Server 2008 administrator seems to suggest that Microsoft testing has found that NLB clusters with more than 8 nodes are not efficient. If you need more than 8 nodes consider multiple NLB clusters which use DNS round robin.

DNS round robin is enabled by default, remember than netmask ordering is enabled by default too; netmask ordering will return the ip address of a particular record which is within the same subnet. DNS netmask ordering uses class C to determine whether the network is local.