Okay, here's what I have so far: GMediaStreamer works in Ubuntu, but a few mp3s don't play and there's no option to control the server like SimpleCenter does. TVersity doesn't work at all [just sits there]. Still haven't tried Twonky yet, but will be testing that later today. At the moment, SimpeCenter works best in Windows so far, and GMediaStreamer works well enough for Ubuntu. There's another Linux UPnP server app that I may give a try later as well. I sure hope MikeL is right about the release of Media Streamer into Maemo Garage. It would be great to see other applications using a similar UI.

Update: Twonky works and has a simple interface, but as of this version, there's no option to control the server. The other Linux UPnP server app turned out not to be a server app but a control point & player instead. So I'm sticking with the SimpleCenter for now. I hope TVersity and others get support for this app soon.