don't forget that they are used to a constant light source when the heat bulbs are on. my 7 week old chicks haven't needed heat for about a week, but i'm still weening them off the "nightlight." when i turned the lights out the first night without heat (they'd been getting heat only at night for a week at that point) they completely piled up, like they would have if they were cold. i knew they weren't cold, since it was the exact temperature they had been at for the last hour or so, so it had to be the light.
as you start to lower the temperature in the brooder or take the light out all together during the day, make sure you give them about 30 mins of darkness each day until they realize that it's nothing to be afraid of. after the light has been out for the 30 mins, i'm moving the beam of light further up the wall each night. in a few days the light will be pointed completely away from the brooder and just giving a little ambient light.