Found a workaround!

It's not perfect, but it's a workaround to the Nvidia driver problem I was having where all the videos turn green.

I started by studying Gmplayer - what makes it different? While looking through the preferences, I notice that it used X11 as the output module. I changed it to "Xv" (which is what I would have expected it to be on) and I got the green video. Changing it back to X11 brought the video back to its colourful best.

I found a similar option in VLC (my favourite DVD player) and now that uses X11 with no problem.

And, to top things off, I found an option in Gconf-editor that tells Gstreamer to use it too. It's a little bit obscure though: First go into gconf-editor, then open up the key "/system/gstreamer/0.10/default". Change the option "videosink" to the value "ximagesink".

If you want to change it back, the original value was "autovideosink".

So now all Gstreamer-based programs, like Elisa and Totem, will display video correctly!

Before you claim that "This means that Linux isn't ready for the desktop", just remember that Nvidia (proprietary software) introduced the problem, and open-source frameworks gave me the workaround.