AMD's proprietary Catalyst driver "FGLRX" uses the X server to set the video mode. This behavior is incompatible with the open source radeon kernel module with attempts to set the video mode using the kernel.
If a user installs Catalyst/FGLRX and kernel mode setting is not disabled for Radeon with radeon.modeset=0 on the kernel line in GRUB, then eventually the entire system will stop booting at the point where X should be loading, and the monitor will suspend itself since nothing is sending it a video signal. At this time, the system also stops responding to the keyboard so ctrl-alt-del or other key combinations to restart the system have no effect.
Since *buntu defaults to no GRUB countdown, it is very difficult to catch it in order to change the kernel boot line to allow it to boot up to where the user can change GRUB's settings. However, the user should not ever reach this point because the FGLRX package should set radeon.modeset=0 for them.
This does not appear to be happening as of now in *buntu Oneiric.
ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: fglrx 2:8.872-0ubuntu1
ProcVersionSignature: Ubuntu 3.0.0-8.11-generic 3.0.1
Uname: Linux 3.0.0-8-generic x86_64
NonfreeKernelModules: fglrx
Architecture: amd64
Date: Tue Aug 16 01:15:26 2011
InstallationMedia: Kubuntu 11.10 "Oneiric Ocelot" - Alpha amd64 (20110803.1)
ProcEnviron:
LANGUAGE=
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: fglrx-installer
UpgradeStatus: No upgrade log present (probably fresh install)
I can confirm this behavior on a new install of Kubuntu Oneiric from today, i have to manually disable modesetting so the system can boot at all.
Ubuntu seems to have this sorted, but Kubuntu still lacks this fix.