Created attachment 137468 [details]
existing xorg.conf

Created attachment 137469 [details]
Failing FC6 Xorg log

Created attachment 137470 [details]
Working FC5 Xorg log

Note how FC5 detects the absence of BIOS and does DDC, identifies the monitors
correctly. And how FC6 thinks there's a Legacy BIOS -- which is rather
surprising on a Mac G5.

This works around it for me, but obviously I wouldn't suggest that it's a proper
fix.

--- xf86-video-ati-6.6.2/src/radeon_bios.c.orig 2006-09-30 12:14:26.000000000 +0100
+++ xf86-video-ati-6.6.2/src/radeon_bios.c      2006-09-30 11:58:34.000000000 +0100
@@ -71,7 +71,7 @@ Bool RADEONGetBIOSInfo(ScrnInfoPtr pScrn
        }
     }
 
-    if (info->VBIOS[0] != 0x55 || info->VBIOS[1] != 0xaa) {
+    if (1 || info->VBIOS[0] != 0x55 || info->VBIOS[1] != 0xaa) {
        xf86DrvMsg(pScrn->scrnIndex, X_WARNING,
                   "Unrecognized BIOS signature, BIOS data will not be used\n");
        xfree (info->VBIOS);

Proper fix at
http://gitweb.freedesktop.org/?p=xorg/driver/xf86-video-ati.git;a=commitdiff;h=c979c7f87fc449cc9631820101edff82c3013e66

About to rebuild and test...

Works correctly on my G5:

(WW) RADEON(0): Not an x86 BIOS ROM image, BIOS data will not be used

Works correctly with a PC version of the Radeon too:

(II) RADEON(0): Legacy BIOS detected

Please also apply
http://gitweb.freedesktop.org/?p=xorg/driver/xf86-video-ati.git;a=commitdiff;h=145c716de6d42778e447581578f3e4db13c9a653

(see bug 207659)

Fixed in 6.6.2-4.