Hello,
I used to have the same problem (i.e. backtracing to the same point) any
time I launched a program that presumably made use of the GLX extension
(I tried glxgears, beryl and stepmania).
xserver-xorg-core was recently updated from 2:1.3.0.0.dfsg-12lenny1 to
2:1.3.0.0.dfsg-12lenny2 . If you built your nvidia module by yourself
(e.g. with the official driver but probably also with module-assistant),
you'll need to re-build it against the new X version so that it operates
correctly.
The error in Xorg's log announcing that the GLX extension could not be
started should disappear and your programs should work again.
Regards,
Xr