Yes, you can put an NFS share on a Win2003 host and have the ESX host see the space as available for use by VMs.
Short instructions:
Download from Microsoft's website and install Windows Services for Unix (SFU) into yoru Win2003 box.
Copy the /etc/passwd and the /etc/group files from the ESX host to the Windows 2003 server using WinSCP (this app is a free download). Then configure user mapping on the Win2003 (this maps, for instance, the root user on the ESX host to the Administrator user on the Win2003 serve and maps the root group to the Administrator group) -- point the User Name Mapping dialogs to your copied over passwd and group files . Then make sure to change the default for User Name Mapping Service on the Win2003 box to Automatic before you restart the Win2003 host (otherwise, the ESX host won't be able to use the NFS space on the Win2003 server).
Create a share on your Windows 2003 server and under Sharing choice, choose the NFS Sharing tab. Choose NFS share name (mount point name), and under the Permissions button, choose read-write access and check the checkbox for Allow Root Access.
On the ESX host, do an Add Storage, choose NFS, input the IP of the Windows 2003 NFS server, the share name (mount point name) and then give the datastore a name.
The NFS share on the Win2003 server NFS share should now show up as a datastore in your ESX server.
Here are more detailed instructions and other forum posts on the same subject:
http://communities.vmware.com/thread/212471
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