Take a look around Google and you can see that people are seriously peeved by the lack of notification LED in the Samsung Galaxy S line of devices (excluding the Epic 4G). Developer and i9000 owner Michael R. - better known as neldar on the XDA forums - was annoyed enough by the glaring omission that he decided to come up with a solution. The result: BLN (BackLight Notification).
The basic idea behind the app is quite simple: as there's no LED on the devices, the logical decision is to light up the soft-touch keys. Doing so isn't quite so simple as the layman might initially think, requiring a modified kernel to be flashed in conjunction with the BLN app.