I have a Lenovo T420 with Intel and Nvidia graphics. On upgrade to Ubuntu Saucy, I had to purge and reinstall bumblebee-nvidia to get beyond optirun failing to find a GPU driver. Now, "optirun glxgears" and "optirun sol" succeed, but optirun fails on 2 Virtual Life viewers that it supported in the past, Cool VL (CoolVLViewer-184.108.40.206-Linux-x86) and Imprudence (Imprudence 1.4.0 beta2). In both cases, the error output is huge, but it starts with
libGL error: failed to load driver: i965
libGL error: failed to load driver: swrast
From the little I can discover, i965 is an Intel graphics driver, which should not be invoked at all. I haven't found any information about swrast. I suspect that some of the X configuration associated with Bumblebee has some Intel dependence that is invoked on certain library calls, but not others. I haven't discovered any definite information on this line.
The Cool VL Viewer runs without optirun, but complains about the insufficiency of the Intel graphics.