I just set up my Linux Mint 10 machine to stream to the WDTV via NFS, and I thought I would post some of the information I used. It's working beautifully, and stuff that was choppy and unplayable over SMB is buttery smooth now. The same information should work for ubuntu, or any Linux distro that supports apt-get or debian.
What you need:
WDTV Gen 1 w/ ethernet adapter and B-RADs WDLXTV firmware
First, follow this guide to set up the NFS exports on your Linux box. No need to repeat it here, as everything you need to know is in this very simple guide:http://havetheknowhow.com/Configure-the-server/Configure-NFS.html
NOTE: The last section that concerns installing the NFS client and mounting the shares is not necessary unless you want to mount the shares on your Linux box.
Next, edit the net.mounts file on your WDTV flash drive. Use a text editor like gedit, vim, etc. Add a line for the share in the following format:
xmount xxx.xxx.xxx.xxx:/path ShareName nfs
If you use the guide above to set up NFS, you will need to include the "exports" folder in the path. This was where I got stuck for a while. For example:
xmount 192.168.1.101:/exports/Media MyMedia nfs
Plug the flash drive back into your WDTV, reboot, and the share should show up as MyMedia, or whatever you entered for the ShareName.
Using NFS from Linux is much more stable and easier to set up than HaneWIN, which crashed all the time, occasionally omitted folders from the share, or just refused to start and required reinstallation.