...for the 3G? I was told by someone that they won't bother with you unless the cracks are coming out of the cut-out areas such as headphone jack, charging plug, volume switch, sim card, and volume knob.
They have also replaced them when they have just hairline cracks down the back. The best way is to just take it in, it does not hurt. I don't know what the policy is for the black one, but there is def. no Official Apple Policy listed online somewhere...