Bring first 4 ingredients to a boil in a large saucepan. Reduce heat, and simmer, stirring occasionally, 5 minutes. Remove from heat.
Add tea bags; cover and steep 5 minutes. Remove tea bags, squeezing gently; add sugar, stirring until dissolved. Cool.
Pour tea through a wire-mesh strainer into a 13- x 9-inch baking dish.
Freeze 8 hours.
Remove mixture from freezer 20 minutes before serving. Process mixture, one-fourth at a time, in a food processor or blender until smooth.
Serve immediately, and garnish, if desired.