So I found that the drm.ko kernel module I built isn't loading properly. It might have something to do with drm being built statically into stock 2.6.28 kernels:
There was also an X server update for Jaunty today (220.127.116.11 -> 18.104.22.1681). I'm going to rebuild DRM, Mesa, and radeonhd and see if anything's improved for the RS780. Will report back.