To change the title of the back button on a UINavigationBar for a given
UIViewController, you'll need to set that on the pushing UIViewController
with the following code.

In this example, this code is written on the InboxViewController, while the
modified title back button will be displayed on MessageViewController.

Note: This can be set and updated at any time.
NSString *newTitle =
[NSString stringWithFormat:@"Inbox (%@)", self.unreadMessagesCount];

UIBarButtonItem *backButtonItem =
[[UIBarButtonItem alloc] initWithTitle:newTitle

self.navigationItem.backBarButtonItem = backButtonItem;
[backButtonItem release];