it's the right "-" key with which you usually open the "Contacts" in Windows Mobile. You have to check the "use center key for start/stop" setting to use the backlight function.
I leave my phone running run.gps in my back pack with the back light turned off and have problems tring to unlock my phone as I can not see the icon very well with the back light off.
Maybe if the Phone \ PDA is in locked mode with the back light off if you tap the screen the back light comes on for around 5 seconds giving you enough time to read your stats on the screen or unlock the device?