the purpose of the pre-op diet is to reduce the fat on the liver, making it easier for the surgeon to place the band (reducing the length of the procedure and other risks).
my surgeon uses Optifast (low calorie, high Protein, low carb, low fat) because, in his words "we know it works every time". the clinic nutritionists work with people who can't tolerate the optifast to devise other programs (slimfast, protein powders, high protein/low fat/low carb diets).
i also asked him, what happens if i eat real food (i have a wedding to attend smack dab in the middle of the pre-op diet phase - i will be on the diet for three weeks) and he advised that he does not expect perfect compliance. he told me to enjoy the meal at the wedding and start back on the shakes the next morning.