I have seen this "sludge" in deceased patients, it can also contain very small sand like grains, which are the beginnings of actual gallstones. So getting the sludge out of the gall bladder is a good thing.
I was talking with a friend today about this subject, he is a medical doctor, but one that believes in natural medicine.
He put out the idea that a very low fat diet, (as is commonly promoted by mainstream medicine), may be responsible for the formation of this gallbladder sludge. Without an adequate intake of good oils/fats, particularly those fats high in plant sterols, the gallbladder does not push out enough bile, (oils and fats cause the gallbladder to contract) and the bile can become stagnant, leading to the formation of this sludge. Its just his theory, but from my own study on this, this theory has merit.