% dpkg -l | grep fglrx
rc  fglrx-kernel-2.6.17-10-generic             8.32.5-1+2.6.17.1-10.34              ATI binary kernel module for Linux 2.6.17-10-g
rc  xorg-driver-fglrx                          8.32.5-1                             Video driver for the ATI graphics accelerators

(just wanted to show that I've uninstalled the xorg-driver-fglrx)

achille@delta ~ ~ 
% sudo apt-get install nvidia-glx
Password:
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Suggested packages:
  nvidia-kernel-source
The following NEW packages will be installed:
  nvidia-glx
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 0B/4066kB of archives.
After unpacking 12.6MB of additional disk space will be used.
(Reading database ... 146707 files and directories currently installed.)
Unpacking nvidia-glx (from .../nvidia-glx_1.0.8776+2.6.17.7-11.1_i386.deb) ...
dpkg-divert: `diversion of /usr/lib/libGL.so.1 to /usr/lib/nvidia/libGL.so.1.xlibmesa by nvidia-glx' clashes with `diversion of /usr/lib/libGL.so.1 to /usr/lib/fglrx/libGL.so.1.xlibmesa by xorg-driver-fglrx'
dpkg: error processing /var/cache/apt/archives/nvidia-glx_1.0.8776+2.6.17.7-11.1_i386.deb (--unpack):
 subprocess pre-installation script returned error exit status 2
Errors were encountered while processing:
 /var/cache/apt/archives/nvidia-glx_1.0.8776+2.6.17.7-11.1_i386.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)