the "S" in S. Darko probably stands for Samantha, as in Donnie's younger sister from the cult classic Donnie Darko. But "S" could also stand for "Sequel". But "sequel" is some times just another word for "cash cow".

Making a sequel for Donnie Darko is like making a sequel for the Matrix, it honestly should not be done. It ended with such finality that viewers are left wanting to know more, but not in a way that actually demands for more, because the unanswered questions are better.

But in about a year, many of your questions will be answered, and you will not like it. The IMDB page says that Samantha and her best friend will have bizarre visions and go on a road trip. Smells like fear and loathing, but probably won't be as cool.