I too have a simuilar set up – NSLU2 feeding a couple of Roku units. I have built an m3u playlist which resiides in the mp3 directory on the NSLU. I have the process m3u option enabled in the .conf file (and I rebooted!)
If I go to the m3u file using something like xmms on a linux box, it will play the mp3 files quite happily, but of course it isn’t streaming them. The m3u playlist doecn’t appear on the Roku units or on itunes on a PC.
Any suggestions please?