On my Debian GNU/Linux 6.0.7 (squeeze) 2.6.32-5-686 I installed the nvidia proprietary driver by hand.
When Debian update itself the OpenGL library required to run glxgears need to be re-linked this way:
(as root:)
# mv /usr/lib/xorg/modules/extensions/libglx.so /usr/lib/xorg/modules/extensions/libglx.so.orig
this make a backup of the current file
# ln -s /usr/lib/xorg/modules/extensions/libglx.so.XXX.YY /usr/lib/xorg/modules/extensions/libglx.so
this link "libglx.so" to the right file "libglx.so.XXX.YY" you have currently installed
XXX.YY is the version number YOU are using (mine is libglx.so.260.19.12)
after this change, X need to be restarted