Catalyst 9.4 (8.602) and 4870x2 with Ubuntu 8.10 32bit seems to be working, i.e. I didn't need any special tricks. I'm assuming (hoping) 8.10 64bit will work too. I'll stick with that until Xorg 1.6 and catalyst 9.4 learn to play better together.
Catalyst 9.7 for linux was released a few weeks ago and I was able to get up to 4 GPUs working with it under a fresh jaunty (9.04) install with my standard fglrx install procedures for Ubuntu 8.10 (no tricky stuff):
CAL Runtime version 1.4.344
Found 4 devices. Using device 0.
Device Type = GPU RV770
Max 1D Width 4294967295.
Max 2D Width 8192.
Max 2D Height 8192.
Found 1024 MB GPU RAM.
Found 625 MHz GPU device clock.
Found 993 MHz GPU memory clock.
Found 10 SIMDs.
Wavefront Size 64 .
Device open and ready.
Found 962 MB available GPU RAM.
Found 1188 MB available uncached remote RAM.
Found 478 MB available cached remote RAM.
Kernel compile and link sucessful.
Device sucessfully closed.
Still no >4GPU support, but this is a definite improvement. Lack of 9.04 support meant I could not use some of my 9.04 machines in GPGPU production. Now I can ... Thanks AMD!