Nolan said it would be last, and Im pretty sure most of the cast only signed for a trilogy, so yeah.
That being said, I sort of hope this doesn't turn true. Its definitely not 100%, since they just say someone on the inside said it, that doesn't always come true. The reason I don't like it is Joker did a lot of Riddler type stuff in TDK. Kind of seems like a rehash, though Nolan will probably make it work. Still, I'd rather see someone fresh, I'd like to see how Nolan would handle one of the monsters instead of the criminals. Monsters being Man Bat, Killer Croc, etc.