My current Samsung Captivate (Galaxy S I) has an app called "BLN control- Free" in the Marketplace that lights up my control buttons whenever I receive an email or some type of notification. Haven't notice a big difference in battery usage from this app either. Hopefully this app runs on the new Galaxy SII phones... anyone know? If this app doesn't run well, is there another one you recommend?
Another one reported in forums is one called "no led". Also blink, blinker, light flow. Remember to read the descriptions though as certain apps might only work for certain phones or have limited features like SMS and call notification only etc etc. I don't have an android phone at the moment so I can't give more specifics nor check out the apps myself.
Battery life does take a significant hit when using BLN. (Supposedly, it prevents the device from entering deep sleep mode.)
As much as I would love to have a notification light, it isn't worth it for me. But do give it a shot and see for yourself.