Stir together first 5 ingredients in a saucepan; bring to a boil over medium heat, stirring constantly. Remove mixture from heat, and stir in soda.
Place half of popcorn and, if desired, nuts in each of 2 lightly greased shallow roasting pans. Pour sugar mixture evenly over popcorn; stir well with a lightly greased spatula.
Bake at 250° for 1 hour, stirring every 15 minutes. Spread on wax paper to cool, breaking apart large clumps as mixture cools. Store in airtight containers.