liquid diet.
I had my DS almost 4 years ago (2/25/09), and I did not have to do a liquid diet, or anything special before surgery.
I've read that some doctors do this so the body will use fat stores in the liver, and reduce the size and density. They want to reduce the chance that they'll damage the liver during surgery.
I had a friend who lost his son during surgery. I was told his son's liver was so full of fat, that when they used retractors, to get to the gallbladder and pancreas, they just went straight through, and tore up his liver.