This may well be the case, as all videos on my archos av500 30gb play with good brightness, even in daylight.
What you can do on many video converting programs is to add brightness to the file (you can in many cases also increase contrast); if you are recording from a tv, you can manually increase the brightness of the recording on the recording settings menu.
My preferred encoder is the 'Pocket DivX Encoder', which specifically converts videos to archos format. This program allows you to increase the brightness aswell.
If it is really that big a problem, id suggest asking for a new one.