I've been researching to do this myself, but I don't think it's possible to have multi-path access from a single VM to a single datastore using NFS.
It's possible with iSCSI using roundrobin (as described above) but with NFS, you would have to define multiple mount points on your ESX servers to alias IP addresses on your NFS server, and then spread out your VM storage across these mounts. So you would have 'multi-path' for multiple VMs, but not for a single VM as with the iSCSI method.
I could be wrong here, if anyone knows a better way with NFS?