From http://www.vmware.com/pdf/vsphere4/r40/vsp_40_esx_server_config.pdf <— look on page 98 or the VMware ESXi Configuration Guide
Network Attached Storage
ESX supports using NAS through the NFS protocol. The NFS protocol enables communication between an NFS client and an NFS server.
The NFS client built into ESX lets you access the NFS server and use NFS volumes for storage. ESX supports only NFS Version 3 over TCP.
You use the vSphere Client to configure NFS volumes as datastores. Configured NFS datastores appear in the vSphere Client, and you can use them to store virtual disk files in the same way that you use VMFS-based datastores.
*** NOTE: ESXi does not support the delegate user functionality that enables access to NFS volumes using non- root credentials.
Also see these links for more info on read only capabilities for different licenses.
http://partnerweb.vmware.com/comp_guide/docs/vSphere_Comp_Matrix.pdf
On the ViOPs site there is a comparison matrix of ESXi/ESX in case we’re asked ‘which one should I use?’.
VMware ESX and ESXi 4.0 Comparison – http://kb.vmware.com/kb/1015000
VMware ESX and ESXi 3.5 Comparison – http://kb.vmware.com/kb/1006543
RCLI is limited to read-only access for the free version of VMware ESXi. To enable full functionality of RCLI on a VMware ESXi host, the host must be licensed with VI Foundation, VI Standard, or VI Enterprise.
http://www.vmware.com/products/vsphere/buy/editions_comparison.html
Comparison of product offerings for vSphere 4.0 and VMware Infrastructure 3.X – http://kb.vmware.com/kb/1010579