From a Q&A on the Rockstar Blog:
Awesome, we love Red Dead too – and are thrilled that so many of you loved John Marston’s tale and are excited for more epic Western action. As we mentioned in our last set of Asked & Answereds to similar questions about the future of other game franchises like Bully and L.A. Noire, we don’t always rush to make sequels but that does not mean that we won’t get to them eventually. Stay tuned for further announcements about the future of the Red Dead series.
Red Dead Redemption is one of the bast games I’ve played in the last 5 years. Way better than GTAIV.
Bully and L.A. Noire can stay done though.