I used to run a very large VMware farm that used all NFS storage. Things like an isolated subnet for IP storage, QoS, and proper storage array performance were critical. I also leveraged the Cisco Nexus platform to provide a class of service specifically for my storage traffic.
My published thoughts on NFS design are here: http://wahlnetwork.com/category/deep-dives/nfs-on-vsphere/
I've also submitted a session (4518) for VMworld on NFS networking design. 