We agree, Slugline on iOS would be great.
But the great news is, it's not strictly necessary. Slugline screenplays can be opened in any text editor, on any platform—iPhone, iPad, or Android. You're not locked in to any one app or family of apps. Use your favorite writing software on whatever mobile platform you like. When you return to Slugline, all your work will be properly formatted. Dropbox makes this especially easy.
Our current favorite iOS app for writing in Fountain is Editorial.