I've been having some problems installing Xubuntu 8.04 and 8.10 on an old IBM ThinkPad X40 I've got here, and I thought I'd share the solution. I initially thought the problem was hardware related, but it was replicated on two identical units, and after I purchased a brand new hard drive for both of them.
The problem was that after installing Xubuntu, the laptop would boot fine once or twice, but after that, boots would fail and I would get the following error:
Could not start the X server (your graphical environment) due to some internal error. Please contact your system administrator or check your syslog to diagnose. In the meantime this display will be disabled. Please restart GDM when the problem is corrected.
I would then be given a CLI log in prompt. Logging in dropped me into a read only file system, and not even sudo would allow me to edit any files.
To skip to the end of this story, I believe that the problem is related to and combination of the drivers in use for the X40 hardware, hot un-docking of the UltraBase docking station, and the JFS file system. JFS is usually my filesystem of choice on Linux, but so far, reinstalling Xubuntu using ext3 seems to have solved the problem.