In our pre 5.1 environment we have multiple clusters without centralized storage.
This is a problem when the cluster nodes need to be upgraded - we can not evacuate the nodes via vMotion since the VMs are on local disk.
Here is a solution: vNFS datastore provided you have adequate local disk on your hosts.
Present an NFS datastore to the cluster via a new VM using local storage.
1 - Configure a Centos/Linux VM with the local storage and share it via NFS
2 - Add the NFS datastore to the cluster nodes
3 - Storage vMotion off the VMs from the node to be upgraded one at a time (monitor the load on the NFS vm via top - the throughput is largely based on the disk IO and secondarily the network)
4 - vMotion off the VMs now they are on the NFS datastore
5 - put the host is maintenance mode and upgrade
6 - reverse the procedure to migrate the VMs back to the upgraded host and local storage
With 5.1 local to local storage vMotion and the upcoming vSAN offering this is less of an issue, but in the interim, its saved us much downtime.