Cut lobster tails in half lengthwise, cutting through upper and lower hard shells with an electric knife. Heat butter in a large skillet over medium heat until butter melts. 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 gingerroot, curry powder, and garlic to skillet. Saute over medium-high heat 2 to 3 minutes. Add half-and-half and next 6 ingredients; stir well. Cook over medium-high heat, stirring constantly, 10 to 12 minutes or 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.