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.

EDIT:

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!

EDIT (N):
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.