Quando instalei o Ubuntu 10.10 (ou qualquer distro derivada do debian), tive um problema semelhante. Eu consegui consertá-lo adicionando a linha abaixo à Seção "Dispositivo" e nomeando o Driver "radeon";
Option "Reverse DDC" "on"
Por exemplo, meu arquivo xorg.conf seria parecido com:
Section "ServerLayout"
Identifier "X.org Configured"
Screen 0 "Screen0" 0 0
InputDevice "Mouse0" "CorePointer"
InputDevice "Keyboard0" "CoreKeyboard"
EndSection
Section "Files"
ModulePath "/usr/lib/xorg/modules"
FontPath "/usr/share/fonts/X11/misc"
FontPath "/usr/share/fonts/X11/cyrillic"
FontPath "/usr/share/fonts/X11/100dpi/:unscaled"
FontPath "/usr/share/fonts/X11/75dpi/:unscaled"
FontPath "/usr/share/fonts/X11/Type1"
FontPath "/usr/share/fonts/X11/100dpi"
FontPath "/usr/share/fonts/X11/75dpi"
FontPath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
FontPath "built-ins"
EndSection
Section "Module"
Load "record"
Load "glx"
Load "dbe"
Load "dri2"
Load "extmod"
Load "dri"
EndSection
Section "InputDevice"
Identifier "Keyboard0"
Driver "kbd"
EndSection
Section "InputDevice"
Identifier "Mouse0"
Driver "mouse"
Option "Protocol" "auto"
Option "Device" "/dev/input/mice"
Option "ZAxisMapping" "4 5 6 7"
EndSection
Section "Monitor"
Identifier "Monitor0"
VendorName "Monitor Vendor"
ModelName "Monitor Model"
EndSection
Section "Device"
Driver "radeon"
### Values: <i>: integer, <f>: float, <bool>: "True"/"False",
### <string>: "String", <freq>: "<f> Hz/kHz/MHz",
### <percent>: "<f>%"
### [arg]: arg optional
#Option "NoAccel" # [<bool>]
#Option "SWcursor" # [<bool>]
#Option "Dac6Bit" # [<bool>]
#Option "Dac8Bit" # [<bool>]
#Option "BusType" # [<str>]
#Option "CPPIOMode" # [<bool>]
#Option "CPusecTimeout" # <i>
#Option "AGPMode" # <i>
#Option "AGPFastWrite" # [<bool>]
#Option "AGPSize" # <i>
#Option "GARTSize" # <i>
#Option "RingSize" # <i>
#Option "BufferSize" # <i>
#Option "EnableDepthMoves" # [<bool>]
#Option "EnablePageFlip" # [<bool>]
#Option "NoBackBuffer" # [<bool>]
#Option "DMAForXv" # [<bool>]
#Option "FBTexPercent" # <i>
#Option "DepthBits" # <i>
#Option "PCIAPERSize" # <i>
#Option "AccelDFS" # [<bool>]
#Option "IgnoreEDID" # [<bool>]
#Option "CustomEDID" # [<str>]
#Option "DisplayPriority" # [<str>]
#Option "PanelSize" # [<str>]
#Option "ForceMinDotClock" # <freq>
#Option "ColorTiling" # [<bool>]
#Option "VideoKey" # <i>
#Option "RageTheatreCrystal" # <i>
#Option "RageTheatreTunerPort" # <i>
#Option "RageTheatreCompositePort" # <i>
#Option "RageTheatreSVideoPort" # <i>
#Option "TunerType" # <i>
#Option "RageTheatreMicrocPath" # <str>
#Option "RageTheatreMicrocType" # <str>
#Option "ScalerWidth" # <i>
#Option "RenderAccel" # [<bool>]
#Option "SubPixelOrder" # [<str>]
#Option "ClockGating" # [<bool>]
#Option "VGAAccess" # [<bool>]
#Option "ReverseDDC" # [<bool>]
#Option "LVDSProbePLL" # [<bool>]
#Option "AccelMethod" # <str>
#Option "DRI" # [<bool>]
#Option "ConnectorTable" # <str>
#Option "DefaultConnectorTable" # [<bool>]
#Option "DefaultTMDSPLL" # [<bool>]
#Option "MacModel" # <str>
#Option "TVDACLoadDetect" # [<bool>]
#Option "ForceTVOut" # [<bool>]
#Option "TVStandard" # <str>
#Option "IgnoreLidStatus" # [<bool>]
#Option "DefaultTVDACAdj" # [<bool>]
#Option "Int10" # [<bool>]
#Option "EXAVSync" # [<bool>]
#Option "ATOMTVOut" # [<bool>]
#Option "R4xxATOM" # [<bool>]
#Option "ForceLowPowerMode" # [<bool>]
#Option "DynamicPM" # [<bool>]
#Option "NewPLL" # [<bool>]
#Option "ZaphodHeads" # <str>
Option "Reverse DDC" "on"
Identifier "Card0"
Driver "radeon"
BusID "PCI:240:16:0"
EndSection
Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
SubSection "Display"
Viewport 0 0
Depth 1
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 4
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 8
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 15
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 16
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 24
EndSubSection
EndSection
Se você não tiver um arquivo xorg (como versões mais recentes do Ubuntu não vêm geralmente com um), pode-se criar um, executando;
Xorg -configure
Isto parece ter feito o truque com todas as distribuições baseadas no Debian que eu já instalei no meu G5. Você pode ver mais detalhes no link abaixo:
Espero que ajude.