SoonerLater,
Not exactly sure which backlight you are talking about, and I say that not because you gave imprecise information, but because I find several backlight-related directories on the X whose brightness files you might be able to play with to see if this helps your issue (or at least helps you identify what might be going on).
I did a "find" for the brightness files and filtered the list by which are contained in backlight-related directories:
#
busybox find / -name 'brightness' 2> /dev/null | busybox grep -i backlight
/sys/devices/platform/i2c_omap.1/i2c-1/1-0038/leds/lcd-backlight/brightness
/sys/devices/platform/ld-keyboard-backlight/leds/keyboard-backlight/brightness
/sys/devices/platform/ld-button-backlight/leds/button-backlight/brightness
So, it looks like there's one for the LCD (screen), buttons, and the keyboard. You can "adb shell" or use a terminal emulator (like Android Terminal Emulator, my personal favorite) to navigate to and play with the settings/values of the brightness file to see if that might turn things off for your.
I played with the
lcd-backlight directory's
brightness file like this:
$
su
#
cd /sys/devices/platform/i2c_omap.1/i2c-1/1-0038/leds/lcd-backlight
#
cat brightness (look at the value before changing)
78
#
echo "0" > brightness (will turn screen off)
#
echo "78" > brightness (will restore to previous value)
There's also a max_brightness file whose value is the usual 255 (highest brightness setting).
Anyway, I've played with the values on my Eris too and they seem to be pretty innocent and simply change the brightness of the associated function's light. I've only tweaked the brightness files, so if you stray from that, you are in uncharted waters, lol.
You might want to play-around with the above to see if there's a value that helps your situation.
If you find that there's a setting that does help you, let me know and I have an idea for something you could use to change these easier than going into your various settings, etc.
Note: while connected via USB/power-source, your screen will not auto-timeout (at least it won't for me

).
Cheers and good luck!