this is just a shot, but try booting with acpi=off
The basic problem. I have a HD6870, my monitor is connected through DVI (& my second one through HDMI).
I've got troubles getting the i915 drivers to work correctly on my laptop (HP Pavilion DM4 2010ea).
Specifically, the laptop screen goes black and stays black after the splash graphic when booting both from USB key and from harddrive.