Booted into 3.18 kernel and X would not start. Booted back into 3.17-4 and all is good.
So, if you're using nvidia 340.46 driver, don't boot into new kernel yet. This could also apply to those using nvidia 343 or 346 drivers (not sure).
# apt-cache policy nvidia-kernel-dkms
nvidia-kernel-dkms:
Installed: 340.46-6
Candidate: 340.46-6
It looks like siduction could package 340.65 in fixes for compatibility with kernel 3.18 or else we'll have to wait for mainstream sid to catch up.
Note: I'm using an 8400GS so a newer 343.xx or 346.xx driver is not an option.