What was the significance of the branded tattoo on Miranda?

It's one of the clues given by the Director for the audience building towards the surprise ending we're treated where its revealed that Talia is the one climbed out of the pits and not Bane. There were many clues which makes sense after the ending including Talia's interest in the Reactor initially during the movie (the scene involving the board and stuff)

There was another scene in the movie where they show a family picture discovered by Selina after Bane takes control of the city . I thought it was a picture of Ra's al ghul family portrait. Then the scene in the court room after Gordon gets the Death by Exile sentence from the Scarecrow, Not sure Talia got the same punishment but Bane asked his goons to send Talia to him.

I think the scar at her back could have been a symbol given by the League of Shadows after she joined in. Bruce was about to get one in Batman Begins which he eventually knocks out to kick start the fire which destroys the house. I think Talia would have gone through a plastic surgery or something to cover it up not to arouse any suspicion from Bruce.

I thought that Bruce noticed the scar on her back. I'm sure he rubbed it with his hand just before the close-up. I also think Miranda made some comment regarding it. ( I only saw this last night, but cannot remember the dialogue..)

It's not exactly the same, but it is very similar to the markings Bane bears on his head (not the ones on the back of his neck, but on his right side of his head toward the back). It's a clue toward their common allegiance.