Now, I wasn't expecting this to be entirely trouble free. My work machine gets all sorts of abuse, with various test versions of anything that might be lying about - so before it started I cleaned up an old zfs beta release, deleted all the different sorts of backup software I had been trying, and tried to clean up and back out all the hacks and kludges.
In the end, none of the problems I was anticipating surfaced. But when it came back up - no graphics. Looking at the
/dev/fb*entries, they looked very suspect.
Turns out this is a known problem with the NVIDIA drivers and the S10U1 upgrade. And it was essentially what I had surmised by looking at the device entries, so a quick deinstall, reinstall, clear up /etc/path_to_inst and a couple of reconfiguration reboots later and everything's back in business again.