 Randomized phase II trial to compare the effectiveness of bortezomib with or without gemcitabine in treating patients who have metastatic pancreatic cancer. Bortezomib may stop the growth of tumor cells by blocking the enzymes necessary for tumor cell growth. Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Combining bortezomib with gemcitabine may kill more tumor cells. 