Storyboard Pro now has spell checker functionality. It checks the spelling of each word in your captions and in your project's script, underlines the words it does not find in its dictionary, and allows you to go through each misspelled word in your project so that you can either correct or ignore it.
By default, automatic spell checking is enabled. To toggle it off, select Caption > Spelling > Check Spelling While Typing from the top menu.
To go through each spelling mistake in your project, or to select which dictionary to use to check your spelling, select Caption > Spelling > Spell Check from the top menu.