"OHAI! You're using 3D acceleration? KER-SPLODY-LOCKUP!"
System hung during X startup. Had to use Alt+SysRq+R to get keyboard control back and switch to another virtual console. /var/log/Xorg.0.log mentioned that fglrx had segfaulted.
Surprisingly, I found helpful people in freenode #ubuntu+1 ... who had me run the command
# apport-cli fglrx
Which stepped me through opening up Ubuntu bug #763130.
Worked around problem by purging all 'fglrx' packages, then using updatedb/locate to find all files on the filesystem containing the string "fglrx", checking to see which of them were actually owned by packages with:
$ for filename in $(locate fglrx); do dpkg-query -S $filename; done
and renaming out every file that wasn't part of a package (the only two that were were:
With all of those renamed out, X started OK (non-accelerated, using the 'radeon' driver)
Some days, my life is a Jerry Pournelle column. <3