I confirmed by reading the drm_init() code that order doesn't matter, and insmod'ing the freshly built files worked fine.
As a footnote, the radeon.ko module logs an init message displaying its version # when it's loaded/probed successfully, before the X server starts. I wasn't even seeing that before, with the old module. But it mostly seems to be working now.
In /var/log/Xorg.log I see a message that AIGLX couldn't init with DRI because of some other missing shared library. I guess there's more I need to rebuild on my Ubuntu 8.10 system. But now Xv is working, and mplayer is using a lot less CPU than before, which is all I really cared about.
(EE) AIGLX error: dlopen of /usr/lib/dri/r600_dri.so failed (/usr/lib/dri/r600_dri.so: cannot open shared object file: No such file or directory) (EE) AIGLX: reverting to software rendering (II) AIGLX: Loaded and initialized /usr/lib/dri/swrast_dri.so