Comment 9 for bug 137626

(In reply to comment #5)
> Changed in the sense that there were no additional options in the section
> Device. I don't have here my notebook so I cannot attach xorg.conf
>
> I didn't understand why 1280x1024 is rejected and 1280xother not, since they
> are not "native" for 15" 4:3 panels.

the only mode that is "native" is 1400x1050. Everything else is scaled. it makes no difference to hw. besides you might want to match your modes to another output device (beamer on a VGA port for example) that supports that mode. The log says why the 1280x1024 modes were rejected:
(II) RADEON(0): Not using default mode "1280x1024" (hsync out of range)
(II) RADEON(0): Not using default mode "1280x1024" (vrefresh out of range)
(II) RADEON(0): Not using default mode "1280x1024" (vrefresh out of range)

>
> PS: I need to use 1280x1024 when I attach the external screen since the maximum
> resolution of the screen is 1280x1024. and if I keep my panel to 1400x1050 the
> image in the external screen is truncated at the bottom and right (because it
> has a smaller resolution)
>

you can add 1280x1024 to the modes line in your screen config or add the mode at runtime using xrandr
xrandr --addmode LVDS 1280x1024