Chop 2 1/4 cucumbers. Purée chopped cucumbers, buttermilk, yogurt, onion and cumin in blender until smooth. Season with salt and pepper. Cover; chill until very cold, at least 4 hours. (Can be made 1 day ahead. Keep refrigerated.)
Finely chop remaining 3/4 cucumber. Mound crabmeat in center of 8 bowls, dividing equally. Ladle cold soup around crabmeat.
Sprinkle with tomato, green onions and finely chopped cucumber.
Per serving: calories, 67; total fat, 0.5 g; saturated fat, 0.5 g; cholesterol, 13 mg.