NFS by itself is insecure...
Usually kerberos is added to the mix. There are a number of tricks you can use though.
Honestly, I only use NFS for exports that don't need any authentication though. I've tested the protocols pretty exensively and Samba, while it does give up some performance when you're using it between unix boxes, it really doesn't give up enough to make adding layers on top of NFS worth fooling with any more. That's just my experience though. I'd love to hear differing opinions.