I'm not sure that this is related to acpid. At least from personal experience, when an application (in your case MythTV) makes you "logout" it's actually crashed the X server, which then automatically restarts and you wind up at the login screen.
Try checking the Xorg log file /var/log/Xorg.0.log (or something similar to that) and see what errors were produced.
Maybe also try searching Google for keywords like "xorg mythtv crash". Some of the results look promising and one indicates that your problem may be related to having transparency enabled.
Try running Mythtv from a console with the following command:
Post back with results