I discovered the hard way that after the upgrade to nvidia-drivers-100.14.09, in order for X to work, I had to modprobe -r nvidia by hand. Now, since the emerge process did not mention this, I naively expected the driver to take care of it (well, to be honest, I had forgotten about the kernel module). It would be helpful if among all the messages at the end of the install there was one to the effect: "Don't forget to unload any old, incompatible nvidia kernel modules before you use this version" to remind people like me who do not keep track of things very well.
*** Bug 207209 has been marked as a duplicate of this bug. ***
added a note.