Is there any way we could just fold this or something like it into the default run environment? I've not fooled with using multiple X servers that much except a brief spate of Xnesting when I attempted to fix a gnome-panel bug back in the day and needed a different X display for gdbing purposes, but it seems like it should be possible to require neither root access nor any manual effort beyond a single commandline argument to runtests.py.
I filed bug 434365[0] to deal with this on Linux, probably with a commandline option. If you have any suggestions, feedback, patches you could drop in there on how to make something like what you suggest "just work", I know a lot of people who'd love to hear it. For anyone in m.d.p.linux, this would be an easy way to make a big impact on development on Linux.
I dithered on where this thread should continue and eventually decided the Linux newsgroup rather than the current one, since the current scheme is only useful on an X-running platform and is hopelessly nonportable to Windows or OS X. If you're not already reading the Linux group, the load's very light, and we could use some fresh blood and enthusiasm there anyway.
Jeff