When I got my N900, the LED wasn't working at all. And some time after, the backlight of the keyboard stopped working.

So I decided to flash the firmware. With absolutely no effect.

I was thinking to send my N900 to Nokia Care, and on the website of nokia care I saw an advice "First thing to do when there is a problem is to remove the battery". That's what I did. And LED and backlight were OK just with a battery removal ! Everything's fine now.