The removal of spent HEU fuel is part of the Global Threat Reduction Initiative program. The GTRI budget for FY2011 included funds for fuel removal from Serbia. The shipment was also part of Russia's commitment to take back Soviet-origin HEU fuel, made at the Nuclear Security Summit in April 2010.
UPDATE 12/22/2010: NNSA announced that the shipment included "13 kg of HEU spent fuel [and] approximately 2.5 metric tons of low-enriched uranium spent fuel." Now that the material has been shipped, no HEU remains in Serbia. According to NNSA, the total amount of material removed from Serbia is "3,031 kilograms of HEU and plutonium."
UPDATE 12/23/2010: The shipment was reported to have reached Mayak on December 22, 2010.