Comment 26 for bug 943263

Same problem with Asus F1A75-M and A8-3870, running stock Oneiric amd64:
monitor went blank almost immediately after boot.
I don't have non-VGA monitor on site so don't know if DVI/HDMI would work.

But disabling modeset like this made text console work:
echo options radeon modeset=0 > /etc/modprobe.d/radeon-kms.conf
(No need for X there at present, so this is sufficient workaround for now.)