So, upgrades happened at home and, as occasionally happens, X stopped working. X stopping working varies from "trivial" to "annoyingly painful" to troubleshoot, especially as there was NOTHING in the xorg.N.log file to indicate what the issue was and it wedged the console to the point where "shutdown, reload" was the only way to get it back (thankfully, I could log on from another machine to do that).
In the end, it was a surprisingly simple fix, after "startx 2>&1 > trace-file" gave me the crucial bits of info. An expected symbol was not around in a dynamic linking stage and chasing that down gave a simple(ish) fix. All I had to do, in the end, was to uninstall the fglrx driver (something I installed in the first place to get working accelerated 3D primitives and direct rendering).
But, it did made me wonder, if the Xorg server can write to stderr, why can't it log the lack of a symbol to the og file? Maybe, I don't know, because that writing happens in a non-X library? I should probably have a poke at that, at some point.