It would seem that someone else has run into the "segfault at X11 startup with radeon and Xinerama" issue that I reported. They reported it against HEAD in March.
Noone has commented on it since then.
There's a new way to do multi-head configurations.
It seems that Xinerama is going to be deprecated some time in the future. XRandR is to replace Xinerama. This is pretty cool, as it simplifies my xorg.conf a *lot*.
This guide tells you everything that you need to know to migrate a single-display adapter setup to XrandrR. NB: the "put this in Xsession.d" advice doesn't work for me. '/etc/X11/xinit/xinitrc.d' is the proper location on my Gentoo system.
That "single-display" thing is a bit of an issue. I have two video cards in this system: a Radeon X850 w/ two monitors and a Voodoo 3 w/ one monitor. ATM, KDE won't permit me to move apps to the the screen with the Voodoo 3. Also, when I enable the V3 screen in my ServerLayout section, the KDE "Display Settings" dialog crashes X.
Hooray. More stuff to figure out!
If I'm reading it correctly, section VI.5 of this document, says that my problem with my Voodoo 3 is a known issue and will be (mostly) addressed with XRandR 1.3. That is, assuming that the tdfx folks integrate the support into their driver.