Okay, so now I'm confused.
I reinstalled planeshift, as an unprivileged user, using the x64 download. The good news, it's now working, and I can log in

Unfortunately, my "broken" install, the one that couldn't load iGraphics3d has also started working. The reinstall was to a different directory, and was carried out as a normal user without root access, so my system libs can't have been touched.
I ran a diff between the two installs using sha1sum, and they-re mostly identical. The sha1sums were different however between some of the updated xml files, and the fresh install. looking a little closer, the xml files were identical, except that the updated ones have the chr(13) carriage return (^M) in vi.
There were differences in psclient.cfg, but that's to be expected -- mainly that the older install had extra stuff like my userid/password and a load of window positions in the config. Also an increased buffer size for alsa.
Anyway, seeing as how both are working, I'm back to using my original, updatered install.
XW