Better than carrots
You have a small flashlight mounted on the left side of your helmet, and in dim conditions it certainly helps, though I often wish it was more powerful, such as when stumbling through the ground section of 'Keyes'. On PAL Xbox it lasts 144 seconds if fully charged. When turned off it seems to recharge at a rate of 4 seconds of charge per second, thus needing 36 seconds to fully recharge after being exhausted.
Leave it off when cloaked
When cloaked, the flashlight is unusable; the beam fades out as you fade out yourself. Nevertheless, remember to leave it off else it can still give you away. If you want to check this for yourself, get cloaked at the start of 'Keyes' then run to the Grunts. Turn your flashlight on and you'll soon have needles and plasma heading your way. For the enemy to notice you however, it seems that you have to be looking sufficiently close to their direction; maybe within 30 degrees either side of them, or something like that (I was testing this with a Jackal, with the first active camouflage you find in level 3). This suggests that the flashlight is sending out light through your cloaking; not enough to be any use, but enough for enemies to notice. Well, either that or there's just a bug in the programming!
Leave it off when creeping up behind someone
Your flashlight can also give you away when you're creeping up behind an enemy (uncloaked). It's as if they can see the light hitting the ground ahead of them. They put two and two together, and turn around to confront you. This is something I first noticed with Grunts in the rockslide megabattle (not quite sure why I had the flashlight on!). It doesn't happen with all enemies in all situations though; watch this space for further info once I've done a bit more investigating.