You might be interested in this post if you are using XFCE and it exits (crashes might be a better word) out to the logon prompt when you try to run an X-Terminal.
This procedure was developed using Xubuntu 7.04 (Feisty Fawn), but should work equally well with any Ubuntu derivative, or Debian derivative for that matter. Also, always be careful when editing your system configuration files, or following any procedures from any site, as you can do harm to your system if a mistake is made. Always make backups of the configuration files that you change, or you might just find yourself with a broken system that's a pain to fix.
This solution is more of a odd work-around than a true fix, so we'll keep an eye out for a resolution in future versions of Xubuntu.
From the Xubuntu login screen press the 'Session' button. Select the 'Failsafe Terminal' option and click the 'Change Session' button in the dialog box that pops up. Then type your Username and Password to log in.
Once you are in 'Failsafe Terminal' mode (you will only see an X Terminal), type sudo vim /etc/X11/xorg.conf and hit Enter. Scroll down until you find the line with DefaultDepth on it. If the entry for the depth on this line is 24, change it to 16 (don't forget to hit the i key to put vim into insert mode). Once you are done changing the depth, hit the escape (Esc) key to exit insert mode (if you haven't already) and hit the colon (:) key to enter command mode. Once vim is in command mode, type wq and hit Enter. The w writes the changes to the file and the q quits vim. At this point you can restart the computer to make the settings take hold (restarting the X server would work too). Make sure to do this restart before trying to launch the X Terminal again.