Yes this annoyed me and believe I just figured it out. The power control widget is necessary to make this work.
To Load the Power Control Widget:
*Press menu>Add>Widgets>Power Control
You must be on one of your three home screens and that screen must have one row with no icons.
The screen brightness button is on the far right of the power control widget.
*Press it repeatedly to cycle through several settings. The bottom button bar's backlight will go out after 5 seconds.
*Quick-Pressing the camera button will re-light the button bar for 5 seconds.
*When the brightness button is at its most dim setting, LONG-Press the screen brightness button until the Droid vibrates, the button bar's backlight is now on solid. You can cycle through the different brightness levels and it will stay lit.
*When the button bar is in "always-on" mode, quick press the camera button and the button bar will go dark in 5 seconds again.
When the screen timeout kicks in and you wake the phone, the backlit buttons will go out again. To start the backlight back up, just cycle through the backlight levels and it should come back on in continuous mode.
Seems to me a smart programmer could make some money with an app that gave us more control over this. Or at least control that was visible rather than hidden with key press sequences.