Cut bananas in half crosswise, and then lengthwise. Toss with lemon juice, and set aside.
Combine margarine and brown sugar in a large skillet; cook over medium heat, stirring constantly, 1 minute or until sugar melts. Add bananas; cook 2 minutes.
Turn bananas, and cook an additional 2 minutes. Sprinkle with cinnamon, and remove from heat. Heat rum in a small saucepan just until warm; remove from heat, and ignite. Pour over banana mixture and let flames die down. Serve immediately over ice milk.