Cut lobster tails in half lengthwise, cutting through upper and lower hard shells with an electric knife. Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add lobster tail halves, cut side down. Cook 4 minutes on each side or until lobster is done. Remove lobster from skillet, and keep warm.
Add margarine and curry powder to skillet; heat over medium-high heat until margarine melts. Cook 30 seconds over medium-high heat, stirring constantly. Add gingerroot and garlic; saute until tender. Add milk and next 6 ingredients; stir well. Cook over medium-high heat, stirring constantly, 10 to 12 minutes until mixture is reduced to 1/2 cup.
Spoon 1/4 cup curry mixture onto each individual serving plate. Place 2 lobster tail halves, cut side down, on each plate. Garnish with green onions, if desired.