Ubuntu 14.04 64-bit.
Graphics card is NVIDIA GTX 670, NVIDIA driver version is 370.23 (beta).
Description of Issue
Startup Blender, switch to Cycles renderer. Segmentation fault (crash).
I have tested and can confirm that this issue does not occur in 2.77a on the same PC.
Oddly enough, 2.78 testbuild2 does not crash when switching to Cycles if you select the camera, go to the Data tab in the Properties editor, switch the camera to 'Panoramic', then switch to Cycles.
However, if you then try to go to the Render tab in the Properties editor, 2.78 testbuild2 will crash again.
It's worth noting that is at least one other crash/segfault I experienced in 2.78 testbuild2 that I do not see in 2.77a, a crash when trying to look at the 'System' tab under 'User Preferences'. (Should I report this as a separate bug?)
Let me know if you have any questions.