Dorothy King has taken on a quite ambitious project-- creating a database of known looted material accessible to the public.
See http://phdiva.blogspot.com/2011/07/database-story-so-far.html
One really wonders why such a project has not been undertaken by source countries like Italy or members of the archaeological community. They certainly have much easier access to such information as well as the resources to make it happen. But, presumably that might ruin the game of "gotcha" these groups and their associated bloggers like to play against collectors and auction houses.
Good luck to Dorothy on this daunting task.