Grumble. I apologize for the noise, I should have been more thorough. Apparently somewhere in the mists of time, while trying to solve things myself, I decided to hand-edit the startx script. And while I did reinstall everything X-related, I apparently did not reinstall xinit itself. A simple sh -x on the startx script showed that it wasn't even trying to run /usr/bin/X... Again, my sincerest apologies for taking your time.