The Thor's Belt is another artifact sought after by Lara in Tomb Raider Underworld. According to what Lara says in the latest gameplay video for Tomb Raider Underworld; "I need Thor's Belt to get his hammer; and I need the hammer to kill a god", Thor's Belt will be used to retrieve Thor's Hammer - the ultimate weapon that could bring forth a catastrophic end to civilization if used.
Lara needs the hammer to destroy the 'god', who is theorized to be Jacqueline Natla who has made an official return to the game franchise. Thor's Belt might just be another artifact, or it could play a significant role like the hammer itself, but it's unlikely. The belt will be found in the Mexico level.