Is that the only error message you're getting? If there are more, post them.
If not, try doing an ldd /usr/lib/libGL.so.1, and seeing if there are any dependencies that it can't find.
What does glxinfo say about "Direct Rendering"?
<obvious question>You did install the GLX half of the driver, correct? </obvious>
The version of NVIDIA_GLX that you used matches your NVIDIA_kernel as well, right?