This problem is discussed here http://ubuntuforums.org/showthread.php?t=1744809
By following the directions you can get the monitor brightness set correctly. Passing the command "setpci -s 00:02.0 F4.B=00 " as root in the dark will cause the screen to light up under BT5. After installing BT5 you can permanently modify your settings, and not have pass the command in the dark.
This is a linux kernel bug. Try any other linux distro with the same kernel and you'll have the same problem. Try FreeBSD, OpenBSD, or an older linux kernel and all is well. It's a major bummer for Acer/emachines owners that want to use BT5 in live mode.