My great-grandfather was a prisoner of war during WWI. He was captured during the the retreat from the Battle of Caporetto in northeast Italy and was a prisoner of war for 14 months in an Austro-Hungarian or German camp. According to family lore, he was so hungry he had to eat his shoes. My great-grandmother's family heard nothing of him after his capture. Her brother was a monseigneur and was able to get someone from the Vatican to track him down and check on him in the camp. When the Germans found out that my great-grandfather was a tailor, they took him off hard labor and had him make clothes for the officers. Soon after he was released, he emigrated to the US.
Someone suggested contacting the International Red Cross: http://www.icrc.org/eng/resources/icrc-archives/index.jsp
Perhaps they have a record of him? It'll take 6-12 months to get a response.
Update 5 Sep 2012
The IRC is digitizing their records. So no requests can be made until 2014.