Dead battery on 2006 Chevrolet Impala SS

On 4/8/10 my battery was dead. I was able to jump start the car and had the battery rechareged and checked out at Merchant's Tire. The battery showed no defects and held a charge. On 6/5/10, the battery died again. I took the car to my Chevy dealer and they were 98% sure that it was an alternator problem, so I told them to put in a new alternator on 6/10/10. The battery was recharged, checked and showed no defects. On 6/25/10, the engine barely turned over. It was the battery problem again. It appears that something is draining the battery, when the ignition is off. Any ideas on what might be draining my battery, when the ignition is off. The car has 28,000 miles.

Asked by for the 2006 Chevrolet Impala SS