I've uploaded OpenGFX to extras-devel. This is the "Free Graphics Replacement" project, and will allow you to play OpenTTD without any non-free files. See http://wiki.openttd.org/Graphics_Replacement .

It won't appear in the application manager though, since my objective is that future OpenTTD versions will silently depend on it, so you can at least launch OpenTTD without any commercial files installed. For now you must install them with apt-get or downloading opengfx_0.1.0alpha4.2_all.deb. You must have openttd (or at least openttd-data) installed.

And btw, you can switch between original or open graphics in the game options screen.